Don't use -fdiagnostics-color=never for rustc

Don't use -fdiagnostics-color=never for rustc

I noticed that the gdb.rust tests fail because the test suite passes
-fdiagnostics-color=never to rustc.  This is not a recognized rustc
option, and the test suite already handles passing the appropriate
option to the Rust compiler.

This patch fixes the problem.

testsuite/ChangeLog
2017-09-05  Tom Tromey  <tom@tromey.com>

	* lib/gdb.exp (gdb_compile): Don't use universal_compile_options
	for rust.