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.

gdb/testsuite/gdb.python/py-unwind.py

@@ -30,7 +30,6 @@ class FrameId(object):
     def pc(self):
         return self._pc
 
-
 class TestUnwinder(Unwinder):
     AMD64_RBP = 6
     AMD64_RSP = 7
@@ -69,6 +68,15 @@ class TestUnwinder(Unwinder):
         This unwinder recognizes the corrupt frames by checking that
         *RBP == RBP, and restores previous RBP from the word above it.
         """
+
+        # Check that we can access the architecture of the pending
+        # frame, and that this is the same architecture as for the
+        # currently selected inferior.
+        inf_arch = gdb.selected_inferior ().architecture ()
+        frame_arch = pending_frame.architecture ()
+        if (inf_arch != frame_arch):
+            raise gdb.GdbError ("architecture mismatch")
+
         try:
             # NOTE: the registers in Unwinder API can be referenced
             # either by name or by number. The code below uses both