Prevent false passes in gdb.base/vla-optimized-out.exp

The "vla_optimized_out" procedure in gdb.base/vla-optimized-out.exp
accepts a "sizeof_result" argument which is substituted into the
regular expression used to check the result of printing the sizeof
a VLA.  The -O3 test variants, however, pass a regular expression
fragment as that argument, which expands into a regular expression
that matches any result with a "6" in it.  This commit wraps the
substitution with parentheses to prevent these false matches.

gdb/testsuite/ChangeLog:

	* gdb.base/vla-optimized-out.exp (p sizeof (a)): Wrap supplied
	regexp fragment in parentheses to prevent false matching.