nvptx/mkoffload.cc: Warn instead of error when reverse offload is not possible

Reverse offload requests at least -misa=sm_35; with this patch, a warning
instead of an error is shown, still permitting reverse offload for all
other configured device types. This is achieved by not calling
GOMP_offload_register_ver (and stopping generating pointless 'static const char'
variables, once known.)

The tool_name as progname changes adds "nvptx " and "gcn " to the
"mkoffload: warning/error:" diagnostic.

gcc/ChangeLog:

	* config/nvptx/mkoffload.cc (process): Replace a fatal_error by
	a warning + not enabling offloading if -misa=sm_30 prevents
	reverse offload.
	(main): Use tool_name as progname for diagnostic.
	* config/gcn/mkoffload.cc (main): Likewise.

libgomp/ChangeLog:

	* libgomp.texi (Offload-Target Specifics: nvptx): Document
	that reverse offload requires >= -march=sm_35.
	* testsuite/libgomp.c-c++-common/requires-4.c: Build for nvptx
	with -misa=sm_35.
	* testsuite/libgomp.c-c++-common/requires-5.c: Likewise.
	* testsuite/libgomp.c-c++-common/requires-6.c: Likewise.
	* testsuite/libgomp.c-c++-common/reverse-offload-1.c: Likewise.
	* testsuite/libgomp.fortran/reverse-offload-1.f90: Likewise.
	* testsuite/libgomp.c/reverse-offload-sm30.c: New test.