amdgcn: Enable OpenACC worker partitioning for AMD GCN

amdgcn: Enable OpenACC worker partitioning for AMD GCN

	gcc/
	* config/gcn/gcn.c (gcn_init_builtins): Override decls for
	BUILT_IN_GOACC_SINGLE_START, BUILT_IN_GOACC_SINGLE_COPY_START,
	BUILT_IN_GOACC_SINGLE_COPY_END and BUILT_IN_GOACC_BARRIER.
	(gcn_goacc_validate_dims): Turn on worker partitioning unconditionally.
	(gcn_fork_join): Update comment.
	* config/gcn/gcn.opt (flag_worker_partitioning): Remove.
	(macc_experimental_workers): Remove unused option.
	libgomp/
	* plugin/plugin-gcn.c (gcn_exec): Change default number of workers to
	16.
	* testsuite/libgomp.oacc-c-c++-common/acc_prof-kernels-1.c
	[acc_device_radeon]: Update.
	* testsuite/libgomp.oacc-c-c++-common/loop-dim-default.c
	[ACC_DEVICE_TYPE_radeon]: Likewise.
	* testsuite/libgomp.oacc-c-c++-common/parallel-dims.c
	[acc_device_radeon]: Likewise.
	* testsuite/libgomp.oacc-c-c++-common/routine-wv-2.c
	[ACC_DEVICE_TYPE_radeon]: Likewise.
	* testsuite/libgomp.oacc-fortran/optional-reduction.f90: XFAIL for
	'openacc_radeon_accel_selected' and '-O0'.
	* testsuite/libgomp.oacc-fortran/reduction-7.f90: Likewise.

Co-Authored-By: Kwok Cheung Yeung <kcy@codesourcery.com>
Co-Authored-By: Thomas Schwinge <thomas@codesourcery.com>