Adjust byte order variable display/change if DW_AT_endianity is present.

- Rationale:
It is possible for compilers to indicate the desired byte order
interpretation of scalar variables using the DWARF attribute:
   DW_AT_endianity

A type flagged with this variable would typically use one of:
   DW_END_big
   DW_END_little
which instructs the debugger what the desired byte order interpretation
of the variable should be.

The GCC compiler (as of V6) has a mechanism for setting the desired byte
ordering of the fields within a structure or union.  For, example, on a
little endian target, a structure declared as:
   struct big {
       int v;
       short a[4];
   } __attribute__( ( scalar_storage_order( "big-endian" ) ) );
could be used to ensure all the structure members have a big-endian
interpretation (the compiler would automatically insert byte swap
instructions before and after respective store and load instructions).

- To reproduce
GCC V8 is required to correctly emit DW_AT_endianity DWARF attributes
in all situations when the scalar_storage_order attribute is used.

A fix for (dwarf endianity instrumentation) for GCC V6-V7 can be found
in the URL field of the following PR:
   https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82509

- Test-case:
A new test case (testsuite/gdb.base/endianity.*) is included with this
patch.

Manual testing for mixed endianity code has also been done with GCC V8.
See:
   https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82509#c4

- Observed vs. expected:

Without this change, using scalar_storage_order that doesn't match the
target, such as

struct otherendian
{
  int v;
} __attribute__( ( scalar_storage_order( "big-endian" ) ) );

would behave like the following on a little endian target:

   Breakpoint 1 at 0x401135: file endianity.c, line 41.
   (gdb) run
   Starting program: /home/pjoot/freeware/t/a.out
   Missing separate debuginfos, use: debuginfo-install glibc-2.17-292.el7.x86_64

   Breakpoint 1, main () at endianity.c:41
   41        struct otherendian o = {3};
   (gdb) n
   43        do_nothing (&o); /* START */
   (gdb) p o
   $1 = {v = 50331648}
   (gdb) p /x
   $2 = {v = 0x3000000}

whereas with this gdb enhancement we can access the variable with the user
specified endianity:

   Breakpoint 1, main () at endianity.c:41
   41        struct otherendian o = {3};
   (gdb) p o
   $1 = {v = 0}
   (gdb) n
   43        do_nothing (&o); /* START */
   (gdb) p o
   $2 = {v = 3}
   (gdb) p o.v = 4
   $3 = 4
   (gdb) p o.v
   $4 = 4
   (gdb) x/4xb &o.v
   0x7fffffffd90c: 0x00    0x00    0x00    0x04

(observe that the 4 byte int variable has a big endian representation in the
 hex dump.)

gdb/ChangeLog
2019-11-21  Peeter Joot  <peeter.joot@lzlabs.com>

	Byte reverse display of variables with DW_END_big, DW_END_little
	(DW_AT_endianity) dwarf attributes if different than the native
	byte order.
	* ada-lang.c (ada_value_binop):
	Use type_byte_order instead of gdbarch_byte_order.
	* ada-valprint.c (printstr):
	(ada_val_print_string):
	* ada-lang.c (value_pointer):
	(ada_value_binop):
	Use type_byte_order instead of gdbarch_byte_order.
	* c-lang.c (c_get_string):
	Use type_byte_order instead of gdbarch_byte_order.
	* c-valprint.c (c_val_print_array):
	Use type_byte_order instead of gdbarch_byte_order.
	* cp-valprint.c (cp_print_class_member):
	Use type_byte_order instead of gdbarch_byte_order.
	* dwarf2loc.c (rw_pieced_value):
	Use type_byte_order instead of gdbarch_byte_order.
	* dwarf2read.c (read_base_type): Handle DW_END_big,
	DW_END_little
	* f-lang.c (f_get_encoding):
	Use type_byte_order instead of gdbarch_byte_order.
	* findvar.c (default_read_var_value):
	Use type_byte_order instead of gdbarch_byte_order.
	* gdbtypes.c (check_types_equal):
	Require matching TYPE_ENDIANITY_NOT_DEFAULT if set.
	(recursive_dump_type): Print TYPE_ENDIANITY_BIG,
	and TYPE_ENDIANITY_LITTLE if set.
	(type_byte_order): new function.
	* gdbtypes.h (TYPE_ENDIANITY_NOT_DEFAULT): New macro.
	(struct main_type) <flag_endianity_not_default>:
	New field.
	(type_byte_order): New function.
	* infcmd.c (default_print_one_register_info):
	Use type_byte_order instead of gdbarch_byte_order.
	* p-lang.c (pascal_printstr):
	Use type_byte_order instead of gdbarch_byte_order.
	* p-valprint.c (pascal_val_print):
	Use type_byte_order instead of gdbarch_byte_order.
	* printcmd.c (print_scalar_formatted):
	Use type_byte_order instead of gdbarch_byte_order.
	* solib-darwin.c (darwin_current_sos):
	Use type_byte_order instead of gdbarch_byte_order.
	* solib-svr4.c (solib_svr4_r_ldsomap):
	Use type_byte_order instead of gdbarch_byte_order.
	* stap-probe.c (stap_modify_semaphore):
	Use type_byte_order instead of gdbarch_byte_order.
	* target-float.c (target_float_same_format_p):
	Use type_byte_order instead of gdbarch_byte_order.
	* valarith.c (scalar_binop):
	(value_bit_index):
	Use type_byte_order instead of gdbarch_byte_order.
	* valops.c (value_cast):
	Use type_byte_order instead of gdbarch_byte_order.
	* valprint.c (generic_emit_char):
	(generic_printstr):
	(val_print_string):
	Use type_byte_order instead of gdbarch_byte_order.
	* value.c (unpack_long):
	(unpack_bits_as_long):
	(unpack_value_bitfield):
	(modify_field):
	(pack_long):
	(pack_unsigned_long):
	Use type_byte_order instead of gdbarch_byte_order.
	* findvar.c (unsigned_pointer_to_address):
	(signed_pointer_to_address):
	(unsigned_address_to_pointer):
	(address_to_signed_pointer):
	(default_read_var_value):
	(default_value_from_register):
	Use type_byte_order instead of gdbarch_byte_order.
	* gnu-v3-abi.c (gnuv3_make_method_ptr):
	Use type_byte_order instead of gdbarch_byte_order.
	* riscv-tdep.c (riscv_print_one_register_info):
	Use type_byte_order instead of gdbarch_byte_order.

gdb/testsuite/ChangeLog
2019-11-21  Peeter Joot  <peeter.joot@lzlabs.com>

	* gdb.base/endianity.c: New test.
	* gdb.base/endianity.exp: New file.

Change-Id: I4bd98c1b4508c2d7c5a5dbb15d7b7b1cb4e667e2

gdb/c-lang.c

@@ -245,7 +245,7 @@ c_get_string (struct value *value, gdb::unique_xmalloc_ptr<gdb_byte> *buffer,
   struct type *element_type = TYPE_TARGET_TYPE (type);
   int req_length = *length;
   enum bfd_endian byte_order
-    = gdbarch_byte_order (get_type_arch (type));
+    = type_byte_order (type);
 
   if (element_type == NULL)
     goto error;