libgomp: Add no-target-region rev offload test + fix plugin-nvptx

OpenMP permits that a 'target device(ancestor:1)' is called without being
enclosed in a target region - using the current device (i.e. the host) in
that case.  This commit adds a testcase for this.

In case of nvptx, the missing on-device 'GOMP_target_ext' call causes that
it and also the associated on-device GOMP_REV_OFFLOAD_VAR variable are not
linked in from nvptx's libgomp.a. Thus, handle the failing cuModuleGetGlobal
gracefully by disabling reverse offload and assuming that the failure is fine.

libgomp/ChangeLog:

	* plugin/plugin-nvptx.c (GOMP_OFFLOAD_load_image): Use unsigned int
	for 'i' to match 'fn_entries'; regard absent GOMP_REV_OFFLOAD_VAR
	as valid and the code having no reverse-offload code.
	* testsuite/libgomp.c-c++-common/reverse-offload-2.c: New test.