amdgcn: OpenMP SIMD routine support

amdgcn: OpenMP SIMD routine support

Enable and configure SIMD clones for amdgcn.  This affects both the __simd__
function attribute, and the OpenMP "declare simd" directive.

Note that the masked SIMD variants are generated, but the middle end doesn't
actually support calling them yet.

gcc/ChangeLog:

	* config/gcn/gcn.cc (gcn_simd_clone_compute_vecsize_and_simdlen): New.
	(gcn_simd_clone_adjust): New.
	(gcn_simd_clone_usable): New.
	(TARGET_SIMD_CLONE_ADJUST): New.
	(TARGET_SIMD_CLONE_COMPUTE_VECSIZE_AND_SIMDLEN): New.
	(TARGET_SIMD_CLONE_USABLE): New.

gcc/testsuite/ChangeLog:

	* gcc.dg/vect/vect-simd-clone-1.c: Add dg-warning.
	* gcc.dg/vect/vect-simd-clone-2.c: Add dg-warning.
	* gcc.dg/vect/vect-simd-clone-3.c: Add dg-warning.
	* gcc.dg/vect/vect-simd-clone-4.c: Add dg-warning.
	* gcc.dg/vect/vect-simd-clone-5.c: Add dg-warning.
	* gcc.dg/vect/vect-simd-clone-8.c: Add dg-warning.