[og9] Fix configury for AMD GCN testing

[og9] Fix configury for AMD GCN testing

	libgomp/
	* plugin/configfrag.ac (amdgcn): Set tgt_plugin.
	* testsuite/lib/libgomp.exp (offload_target_to_openacc_device_type):
	Add AMD GCN support.
	(check_effective_target_openacc_amdgcn_accel_selected): Test
	offload_target instead of offload_target_openacc.
	* testsuite/libgomp.oacc-c++/c++.exp (amdgcn*): Rename stanza to...
	(gcn): ...this. Don't set tagopt redundantly here.
	* testsuite/libgomp.oacc-c/c.exp (amdgcn*, gcn): Likewise.
	* testsuite/libgomp.oacc-fortran/fortran.exp (amdgcn*, gcn): Likewise.
	* configure: Regenerated.

(cherry picked from openacc-gcc-9-branch commit
a48bf635e1b7ef94a057ed10d433070ccf5724de)