GDB testsuite: Suppress GCC's colored output

GDB testsuite: Suppress GCC's colored output

Newer GCC versions yield colored diagnostic messages by default, which may
be useful when executing GDB interactively from a terminal.  But when run
from a GDB test case, the compiler output is written into gdb.log, where
such escape sequences are usually more inhibiting than helpful to the
evaluation of test results.  So this patch suppresses that.

gdb/testsuite/ChangeLog:

	* lib/gdb.exp (universal_compile_options): New caching proc.
	(gdb_compile): Suppress GCC's coloring of messages.