libgomp/testsuite: Fix checks for dg-excess-errors

libgomp/testsuite: Fix checks for dg-excess-errors

For the tests modified below, the effective target line has to be effective
when compiling for an offload target, except that variable-not-offloaded.c
would compile with unified-share memory and pr86416-*.c if long double/float128
is supported.
The previous check used a run-time device ability check. This new variant
now enables those dg- lines when _compiling_ for nvptx or gcn.

libgomp/ChangeLog:

	* testsuite/lib/libgomp.exp (offload_target_to_openacc_device_type):
	New, based on check_effective_target_offload_target_nvptx.
	(check_effective_target_offload_target_nvptx): Call it.
	(check_effective_target_offload_target_amdgcn): New.
	* testsuite/libgomp.c-c++-common/function-not-offloaded.c:
	Require target offload_target_nvptx || offload_target_amdgcn.
	* testsuite/libgomp.c-c++-common/variable-not-offloaded.c: Likewise.
	* testsuite/libgomp.c/pr86416-1.c: Likewise.
	* testsuite/libgomp.c/pr86416-2.c: Likewise.