GCN, nvptx: 'sorry, unimplemented: exception handling not supported'

For GCN, this avoids ICEs further down the compilation pipeline.  For nvptx,
there's effectively no change: in presence of exception handling constructs,
instead of 'sorry, unimplemented: target cannot support nonlocal goto', we
now emit 'sorry, unimplemented: exception handling not supported'.

Additionally, turn test cases into UNSUPPORTED if running into
'sorry, unimplemented: exception handling not supported'.

	gcc/
	* config/gcn/gcn.md (exception_receiver): 'define_expand'.
	* config/nvptx/nvptx.md (exception_receiver): Likewise.
	gcc/testsuite/
	* lib/gcc-dg.exp (gcc-dg-prune): Turn
	'sorry, unimplemented: exception handling not supported' into
	UNSUPPORTED.
	* gcc.dg/pr104464.c: Remove GCN XFAIL.
	libstdc++-v3/
	* testsuite/lib/prune.exp (libstdc++-dg-prune): Turn
	'sorry, unimplemented: exception handling not supported' into
	UNSUPPORTED.