Add global_context parameter to gdb.parse_and_eval

This adds a 'global_context' parse_and_eval to gdb.parse_and_eval.
This lets users request a parse that is done at "global scope".

I considered letting callers pass in a block instead, with None
meaning "global" -- but then there didn't seem to be a clean way to
express the default for this parameter.

Reviewed-By: Eli Zaretskii <eliz@gnu.org>