[libgomp, testsuite] Don't shadow global 'offload_targets' variable

See local 'offload_targets' variable in
'libgomp/testsuite/lib/libgomp.exp:libgomp_check_effective_target_offload_target'
vs. global 'libgomp/testsuite/libgomp-test-support.exp.in:offload_targets'
variable.

	libgomp/
	* testsuite/lib/libgomp.exp
	(check_effective_target_offload_target_nvptx): Don't shadow global
	'offload_targets' variable.