[nvptx, testsuite] Add -mptx=_ in nvptx.exp test-cases

[nvptx, testsuite] Add -mptx=_ in nvptx.exp test-cases

When running with target board nvptx-none-run/-mptx=3.1, I run into:
...
cc1: error: PTX version (-mptx) needs to be at least 4.2 to support selected \
  -misa (sm_53)^M
compiler exited with status 1
FAIL: gcc.target/nvptx/atomic-store-1.c (test for excess errors)
...

Fix this and similar cases by adding an explicit -mptx=_ setting.

Tested on nvptx.

gcc/testsuite/ChangeLog:

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

	* gcc.target/nvptx/atomic-store-1.c: Add -mptx=_.
	* gcc.target/nvptx/atomic-store-2.c: Same.
	* gcc.target/nvptx/float16-1.c: Same.
	* gcc.target/nvptx/float16-2.c: Same.
	* gcc.target/nvptx/float16-3.c: Same.
	* gcc.target/nvptx/float16-4.c: Same.
	* gcc.target/nvptx/float16-5.c: Same.
	* gcc.target/nvptx/float16-6.c: Same.
	* gcc.target/nvptx/tanh-1.c: Same.
	* gcc.target/nvptx/uniform-simt-1.c: Same.
	* gcc.target/nvptx/uniform-simt-3.c: Same.