GDB testsuite: Re-enable -fdiagnostics-color=never

GDB testsuite: Re-enable -fdiagnostics-color=never

In August 2017 the GDB test suite was changed to always add the compile
option "-fdiagnostics-color=never", see:

  https://sourceware.org/ml/gdb-patches/2017-08/msg00150.html

Since this option is not understood by rustc, a commit from 09/2017
dropped its use in that case:

  https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;a=commit;h=5eb5f850
  ("Don't use -fdiagnostics-color=never for rustc")

But that change goes overboard and stops using the option for other
languages as well.  Thus compiler diagnostics written into gdb.log may
contain colored output again.  This is fixed.

gdb/testsuite/ChangeLog:

	* lib/gdb.exp (gdb_compile): Re-enable use of
	universal_compile_options for languages other than Rust.