gcc/
	* config/i386/x86-tune.def (X86_TUNE_AVX256_OPTIMAL): Add tuning
	option prefer-avx256 for skylake-avx512 configuration.
	* config/i386/i386.c (ix86_option_override_internal): Ditto.
	(get_builtin_code_for_version): Ditto.

From-SVN: r254855