Make inf_ptrace_trad Linux-only, move to separate file

There are only two inf_ptrace_trad_target users, MIPS GNU/Linux and
Alpha GNU/Linux.  They both call it via linux_trad_target.

Move this code out of inf-ptrace.c to a GNU/Linux-specific new file.

Making this code be GNU/Linux-specific simplifies C++ification of
target_ops, because we can make the trad target inherit linux_nat
instead of inheriting inf_ptrace.  That'll be visible in a later patch.

Note this makes linux_target_install_ops an extern function, but that
is temporary -- the function will disappear once target_ops is made a
C++ class with virtual methods, later in the series.  Also, I did not
rename the functions in the new file for a similar reason.  They'll be
renamed again anyway in a couple of patches.

gdb/ChangeLog:
2018-05-02  Pedro Alves  <palves@redhat.com>

	* alpha-linux-nat.c: Include "linux-nat-trad.h" instead of
	"linux-nat.h".
	* configure.nat (alpha-linux, linux-mips): Add linux-nat-trad.o.
	* inf-ptrace.c (inf_ptrace_register_u_offset)
	(inf_ptrace_fetch_register, inf_ptrace_fetch_registers)
	(inf_ptrace_store_register, inf_ptrace_store_registers)
	(inf_ptrace_trad_target): Move to ...
	* linux-nat-trad.c: ... this new file.
	* linux-nat-trad.h: New file.
	* linux-nat.c (linux_target_install_ops): Make extern.
	(linux_trad_target): Delete.
	* linux-nat.h (linux_trad_target): Delete declaration.
	(linux_target_install_ops): Declare.
	* mips-linux-nat.c: Include "linux-nat-trad.h" instead of
	"linux-nat.h".

gdb/alpha-linux-nat.c

@@ -19,7 +19,7 @@
 #include "defs.h"
 #include "target.h"
 #include "regcache.h"
-#include "linux-nat.h"
+#include "linux-nat-trad.h"
 
 #include "alpha-tdep.h"