[libgomp, testsuite, nvptx] Add -mptx=_ in declare-variant-3-sm*.c

[libgomp, testsuite, nvptx] Add -mptx=_ in declare-variant-3-sm*.c

When running with target board unix/-foffload=-mptx=3.1, we run into:
...
lto1: error: PTX version (-mptx) needs to be at least 4.2 to support \
  selected -misa (sm_53)^M
mkoffload: fatal error: x86_64-pc-linux-gnu-accel-nvptx-none-gcc returned \
  1 exit status^M
compilation terminated.^M
  ...
FAIL: libgomp.c/declare-variant-3-sm53.c (test for excess errors)
...

Fix this by adding -foffload=-mptx=_ in the libgomp.c/declare-variant-3-sm*.c
test-cases.

Tested on x86_64 with nvptx accelerator.

libgomp/ChangeLog:

2022-02-28  Tom de Vries  <tdevries@suse.de>

	* testsuite/libgomp.c/declare-variant-3-sm30.c: Add -foffload=-mptx=_.
	* testsuite/libgomp.c/declare-variant-3-sm35.c: Same.
	* testsuite/libgomp.c/declare-variant-3-sm53.c: Same.
	* testsuite/libgomp.c/declare-variant-3-sm70.c: Same.
	* testsuite/libgomp.c/declare-variant-3-sm75.c: Same.
	* testsuite/libgomp.c/declare-variant-3-sm80.c: Same.