gdb/python: Add architecture method to gdb.PendingFrame

gdb/python: Add architecture method to gdb.PendingFrame

It could be useful to determine the architecture of a frame being
unwound during the frame unwind process, that is, before we have a
gdb.Frame, but when we only have a gdb.PendingFrame.

The PendingFrame already has a pointer to the gdbarch internally, this
commit just exposes an 'architecture' method to Python, and has this
return a gdb.Architecture object (list gdb.Frame.architecture does).

gdb/ChangeLog:

	* NEWS: Mention new Python API method.
	* python/py-unwind.c (pending_framepy_architecture): New function.
	(pending_frame_object_methods): Add architecture method.

gdb/testsuite/ChangeLog:

	* gdb.python/py-unwind.py (TestUnwinder::__call__): Add test for
	gdb.PendingFrame.architecture method.

gdb/doc/ChangeLog:

	* python.texi (Unwinding Frames in Python): Document
	PendingFrame.architecture method.