Fix architecture support in OMP_OFFLOAD_init_device for gcn

The following makes the existing architecture support check work
instead of being optimized away (enum vs. -1).  This avoids
later asserts when we assume such devices are never actually
used.

libgomp/
	* plugin/plugin-gcn.c
	(EF_AMDGPU_MACH::EF_AMDGPU_MACH_UNSUPPORTED): Add.
	(isa_code): Return that instead of -1.
	(GOMP_OFFLOAD_init_device): Adjust.