[nvptx, testsuite] Xfail stack atomics

[nvptx, testsuite] Xfail stack atomics

Nvptx currently doesn't support atomics if it involves a stack address.
PR97444 is open about.

Xfail corresponding execution fails in gcc.dg/atomic.  Tested on nvptx.

gcc/testsuite/ChangeLog:

2020-10-15  Tom de Vries  <tdevries@suse.de>

	* gcc.dg/atomic/c11-atomic-exec-6.c: Xfail execution for nvptx.
	* gcc.dg/atomic/c11-atomic-exec-7.c: Same.
	* gcc.dg/atomic/stdatomic-op-5.c: Same.