ctf: Do not warn for CTF not supported for GNU GIMPLE

CTF is supported for C only.  Currently, a warning is emitted if the -gctf
command line option is specified for a non-C frontend.  This warning is also
used by the GCC testsuite framework - it skips adding -gctf to the list of
debug flags for automated testing, if CTF is not supported for the frontend.

The following warning, however, is not useful in case of LTO:

"lto1: note: CTF debug info requested, but not supported for ‘GNU GIMPLE’
frontend"

This patch disables the generation of the above warning for GNU GIMPLE.

gcc/ChangeLog:

	* toplev.c (process_options): Do not warn for GNU GIMPLE.