Add test cases for exception handling constructs in dead code for GCN, nvptx target...

Add test cases for exception handling constructs in dead code for GCN, nvptx target and OpenMP 'target' offloading [PR118794]

	PR target/118794
	gcc/testsuite/
	* g++.target/gcn/exceptions-pr118794-1.C: New.
	* g++.target/nvptx/exceptions-pr118794-1.C: Likewise.
	libgomp/
	* testsuite/libgomp.c++/target-exceptions-pr118794-1.C: New.
	* testsuite/libgomp.c++/target-exceptions-pr118794-1-offload-sorry-GCN.C:
	Likewise.
	* testsuite/libgomp.c++/target-exceptions-pr118794-1-offload-sorry-nvptx.C:
	Likewise.