libgomp: Add offload_device_gcn check, add requires-4a.c test

libgomp: Add offload_device_gcn check, add requires-4a.c test

Duplicate libgomp.c-c++-common/requires-4.c (as ...-4a.c) but
with using a heap-allocated instead of static memory for a variable.

This change and the added offload_device_gcn check prepare for
pseudo-USM, where the device hardware cannot access all host
memory but only managed and pinned memory; for those, requires-4.c
will fail and the new check permits to add
  target { ! { offload_device_nvptx || offload_device_gcn } }
to requires-4.c; however, it has not been added yet as pseuo-USM
support is not yet on mainline. (Review is pending for the USM
patches.)

include/ChangeLog:

	* gomp-constants.h (GOMP_DEVICE_HSA): Comment out unused define.

libgomp/ChangeLog:

	* testsuite/lib/libgomp.exp (check_effective_target_offload_device_gcn):
	New.
	* testsuite/libgomp.c-c++-common/on_device_arch.h (device_arch_gcn,
	on_device_arch_gcn): New.
	* testsuite/libgomp.c-c++-common/requires-4a.c: New test; copied from
	requires-4.c but using heap-allocated memory.