Add Inferior.architecture method

Add Inferior.architecture method

I've written a couple of gdb unwinders in Python, and while doing so,
I wanted to find the architecture of the inferior.  (In an unwinder in
particular, one can't use the frame's architecture, because there is
no frame.)

This patch adds Inferior.architecture to allow this.  Normally I think
I would have chosen an attribute and not a method here, but seeing
that Frame.architecture is a method, I chose a method as well, for
consistency.

gdb/ChangeLog
2018-10-06  Tom Tromey  <tom@tromey.com>

	PR python/19399:
	* python/py-inferior.c: Add "architecture" entry.
	(infpy_architecture): New function.

gdb/doc/ChangeLog
2018-10-06  Tom Tromey  <tom@tromey.com>

	PR python/19399:
	* python.texi (Inferiors In Python): Document
	Inferior.Architecture.

gdb/testsuite/ChangeLog
2018-10-06  Tom Tromey  <tom@tromey.com>

	PR python/19399:
	* gdb.python/py-inferior.exp: Add architecture test.