[nvptx] Add -mptx=_ in gcc.target/nvptx/smxx.c

[nvptx] Add -mptx=_ in gcc.target/nvptx/smxx.c

With target board nvptx-none-run/-mptx=3.1 we 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/sm53.c (test for excess errors)
...

Fix this by adding -mptx=_ in sm53.c and similar.

Tested on nvptx.

gcc/testsuite/ChangeLog:

2022-03-03  Tom de Vries  <tdevries@suse.de>

	* gcc.target/nvptx/sm53.c: Add -mptx=_.
	* gcc.target/nvptx/sm70.c: Same.
	* gcc.target/nvptx/sm75.c: Same.
	* gcc.target/nvptx/sm80.c: Same.