Fix 'libgomp.c/declare-variant-3-*.c' compilation for configurations where GCN offloading is enabled in addition to nvptx
The GCN offloading compiler doesn't like '-misa=sm_30' etc.; restrict to
'-foffload=nvptx-none' compilation only.
Fix-up for commit
59b8ade88774b4dcf1691a8f650cdbb86cc30862
"[libgomp, testsuite, nvptx] Add libgomp.c/declare-variant-3-sm*.c".
libgomp/
* testsuite/libgomp.c/declare-variant-3-sm30.c:
'dg-additional-options -foffload=nvptx-none'.
* testsuite/libgomp.c/declare-variant-3-sm35.c: Likewise.
* testsuite/libgomp.c/declare-variant-3-sm53.c: Likewise.
* testsuite/libgomp.c/declare-variant-3-sm70.c: Likewise.
* testsuite/libgomp.c/declare-variant-3-sm75.c: Likewise.
* testsuite/libgomp.c/declare-variant-3-sm80.c: Likewise.