[nvptx] Improve gcc.target/nvptx/atomic_fetch-*.c test-cases

[nvptx] Improve gcc.target/nvptx/atomic_fetch-*.c test-cases

Fix a few issues in test-cases gcc.target/nvptx/atomic_fetch-*.c:
- atomic_fetch-1.c uses scan-assembler instead of scan-assembler-times,
  which is less accurate
- atomic_fetch-2.c only contains negative testing using
  scan-assembler-not
- the test-cases use stack variables to generate generic addresses,
  while stack atomics are not natively supported
- the test-cases only test (64-bit) x (generic), instead of
  (32-bit, 64-bit) x (generic, global, shared)
- the test-cases use a hardcoded '0' instead of the clearer
  MEMMODEL_RELAXED

Tested on nvptx.

gcc/testsuite/ChangeLog:

2022-01-12  Tom de Vries  <tdevries@suse.de>

	* gcc.target/nvptx/atomic_fetch-1.c: Rewrite.
	* gcc.target/nvptx/atomic_fetch-2.c: Rewrite.