[gcn] Use 'radeon' for the environment variable 'ACC_DEVICE_TYPE'

..., per OpenACC 3.0, A.1.2. "AMD GPU Targets".

This complements commit 6687d13a "Rename
acc_device_gcn to acc_device_radeon".

	libgomp/
	* oacc-init.c (get_openacc_name): Handle 'gcn'.
	* testsuite/lib/libgomp.exp
	(offload_target_to_openacc_device_type) [amdgcn*]: Return
	'radeon'.  Adjust all users.
	(check_effective_target_openacc_amdgcn_accel_present): Rename
	to...
	(check_effective_target_openacc_radeon_accel_present): ... this.
	Adjust all users.
	(check_effective_target_openacc_amdgcn_accel_selected): Rename to...
	(check_effective_target_openacc_radeon_accel_selected): ... this.
	Adjust all users.