GDB testsuite: Escape paths used in regular expressions

This patch fixes problems with a few GDB testsuites when executing in a
path that contains special characters (e.g. "++").  When such paths are
used as a regular expression, the regular expression parser will choke
and cause the tests to fail.  This patch uses string_to_regexp to
escape strings that will be used as regular expressions, in order to
sanitize path names used in expect scripts.

2016-07-15  Zachary Welch  <zwelch@codesourcery.com>
	    Don Breazeal <donb@codesourcery.com>

	gdb/testsuite/ChangeLog:
	* gdb.base/maint.exp: Escape paths used in regular expressions.
	* gdb.stabs/weird.exp: Likewise.