[og9] A couple of GCN-specific test fixes

[og9] A couple of GCN-specific test fixes

	libgomp/
	* testsuite/libgomp.oacc-c-c++-common/async_queue-1.c: Only run
	NVidia-specific test on NVidia hardware.
	* testsuite/libgomp.oacc-c-c++-common/asyncwait-nop-1.c (main):
	Initialise for acc_device_gcn if testing on AMD GCN.
	* testsuite/libgomp.oacc-c-c++-common/function-not-offloaded.c: Support
	AMD GCN.
	* testsuite/libgomp.oacc-c-c++-common/loop-dim-default.c (check): Skip
	vector dimension test for AMD GCN.

(cherry picked from openacc-gcc-9-branch commit
0467f885d40e76d2d34dac372e4b3dd9b84c31f1)