MI: Fix printing of frame architecture with Python frame filters enabled

MI: Fix printing of frame architecture with Python frame filters enabled

Commit 6d52907e226a (MI: Print frame architecture when printing frames
on an MI channel) added frame's architecture to MI frame output. However
the frame architecture was not correctly printed in the output of
"-stack-list-frames" with frame filters enabled (via "-enable-frame-filters").
This was because with frame filters enabled, the actual frame printing is
done in "py_print_frame" rather than "print_frame". This issue is now fixed.

gdb/Changelog:
2018-08-27  Jan Vrany  <jan.vrany@fit.cvut.cz>

	* python/py-framefilter.c (py_print_frame): Print frame architecture
	when printing on an MI output.

gdb/testsuite/Changelog:
2018-08-27  Jan Vrany  <jan.vrany@fit.cvut.cz>

	* gdb.python/py-framefilter-mi.exp: Update regexp to
	check for "arch" field in frame output.