Add __attribute__((__simd__)) to GCC.

Add __attribute__((__simd__)) to GCC.

gcc/
	* omp-low.c (pass_omp_simd_clone::gate): If target allows - call
	without additional conditions.
	* doc/extend.texi (@item simd): New.
gcc/c-family/
	* c-common.c (handle_simd_attribute): New.
	(struct attribute_spec): Add entry for "simd".
	(handle_simd_attribute): New.
gcc/c/
	* c-parser.c (c_finish_omp_declare_simd): Look for
	"simd" attribute as well. Update error message.
gcc/cp/
	* parser.c (cp_parser_late_parsing_cilk_simd_fn_info): Look for
	"simd" attribute as well. Update error message.
gcc/testsuite/
	* c-c++-common/attr-simd.c: New test.
	* c-c++-common/attr-simd-2.c: New test.
	* c-c++-common/attr-simd-3.c: New test.

From-SVN: r230422