gdb/python: Add gdb.Architecture.registers method

This commit adds a new method gdb.Architecture.registers that returns
an object of the new type gdb.RegisterDescriptorIterator.  This
iterator returns objects of the new type gdb.RegisterDescriptor.

A RegisterDescriptor is not a way to read the value of a register,
this is already covered by Frame.read_register, a RegisterDescriptor
is simply a way to discover from Python, which registers are
available for a given architecture.

I did consider just returning a string, the name of each register,
instead of a RegisterDescriptor, however, I'm aware that it we don't
want to break the existing Python API in any way, so if I return just
a string now, but in the future we want more information about a
register then we would have to add a second API to get that
information.  By going straight to a descriptor object now, it is easy
to add additional properties in the future should we wish to.

Right now the only property of a register that a user can access is
the name of the register.

In future we might want to be able to ask the register about is
register groups, or its type.

gdb/ChangeLog:

	* Makefile.in (SUBDIR_PYTHON_SRCS): Add py-registers.c
	* python/py-arch.c (archpy_registers): New function.
	(arch_object_methods): Add 'registers' method.
	* python/py-registers.c: New file.
	* python/python-internal.h
	(gdbpy_new_register_descriptor_iterator): Declare.
	(gdbpy_initialize_registers): Declare.
	* python/python.c (do_start_initialization): Call
	gdbpy_initialize_registers.
	* NEWS: Mention additions to the Python API.

gdb/testsuite/ChangeLog:

	* gdb.python/py-arch-reg-names.exp: New file.

gdb/doc/ChangeLog:

	* python.texi (Python API): Add new section the menu.
	(Frames In Python): Add new @anchor.
	(Architectures In Python): Document new registers method.
	(Registers In Python): New section.

gdb/python/python.c

@@ -1759,6 +1759,7 @@ do_start_initialization ()
       || gdbpy_initialize_py_events () < 0
       || gdbpy_initialize_event () < 0
       || gdbpy_initialize_arch () < 0
+      || gdbpy_initialize_registers () < 0
       || gdbpy_initialize_xmethods () < 0
       || gdbpy_initialize_unwind () < 0
       || gdbpy_initialize_tui () < 0)