Initial support for -mevex512

Initial support for -mevex512

gcc/ChangeLog:

	* common/config/i386/i386-common.cc
	(OPTION_MASK_ISA2_EVEX512_SET): New.
	(OPTION_MASK_ISA2_EVEX512_UNSET): Ditto.
	(ix86_handle_option): Handle EVEX512.
	* config/i386/i386-c.cc
	(ix86_target_macros_internal): Handle EVEX512. Add __EVEX256__
	when AVX512VL is set.
	* config/i386/i386-options.cc: (isa2_opts): Handle EVEX512.
	(ix86_valid_target_attribute_inner_p): Ditto.
	(ix86_option_override_internal): Set EVEX512 target if it is not
	explicitly set when AVX512 is enabled. Disable
	AVX512{PF,ER,4VNNIW,4FAMPS} for -mno-evex512.
	* config/i386/i386.opt: Add mevex512. Temporaily RejectNegative.