nvptx: In mkoffload.cc, call diagnostic_color_init + gcc_init_libintl: Restore 'libgo...

nvptx: In mkoffload.cc, call diagnostic_color_init + gcc_init_libintl: Restore 'libgomp.c/reverse-offload-sm30.c' testing

With commit 7520a4992c94254016085a461c58c972497c4483
"nvptx: In mkoffload.cc, call diagnostic_color_init + gcc_init_libintl",
we regressed:

    [-PASS:-]{+FAIL:+} libgomp.c/reverse-offload-sm30.c  at line 15 (test for warnings, line )
    [-PASS:-]{+FAIL:+} libgomp.c/reverse-offload-sm30.c (test for excess errors)

	libgomp/
	* testsuite/libgomp.c/reverse-offload-sm30.c: Set 'GCC_COLORS' to the empty string.