libgomp: plugin-gcn - support 'unified_address'

Effectively, for GCN (as for nvptx) there is a common address space between
host and device, whether being accessible or not. Thus, this commit
permits to use 'omp requires unified_address' with GCN devices.
(nvptx accepts this requirement since r13-3460-g131d18e928a3ea.)

libgomp/

	* plugin/plugin-gcn.c (GOMP_OFFLOAD_get_num_devices): Regard
	unified_address requirement as supported.
	* libgomp.texi (OpenMP 5.0, AMD Radeon, nvptx): Remove
	'unified_address' from the not-supported requirements.