[nvptx] Use _ as destination operand of atom.exch

[nvptx] Use _ as destination operand of atom.exch

We currently generate this code for an atomic store:
...
.reg.u32 %r21;
atom.exch.b32 %r21,[%r22],%r23;
...
where %r21 is set but unused.

Use the ptx bit bucket operand '_' instead, such that we have:
...
atom.exch.b32 _,[%r22],%r23;
...

[ Note that the same problem still occurs for this code:
...
void atomic_store (int *ptr, int val) {
  __atomic_exchange_n (ptr, val, MEMMODEL_RELAXED);
}
... ]

Tested on nvptx.

gcc/ChangeLog:

2022-02-19  Tom de Vries  <tdevries@suse.de>

	* config/nvptx/nvptx.cc (nvptx_reorg_uniform_simt): Handle SET insn.
	* config/nvptx/nvptx.md
	(define_insn "nvptx_atomic_store<mode>"): Rename to ...
	(define_insn "nvptx_atomic_store_sm70<mode>"): This.
	(define_insn "nvptx_atomic_store<mode>"): New define_insn.
	(define_expand "atomic_store<mode>"): Handle rename.  Use
	nvptx_atomic_store instead of atomic_exchange.

gcc/testsuite/ChangeLog:

2022-02-19  Tom de Vries  <tdevries@suse.de>

	* gcc.target/nvptx/atomic-store-1.c: Update.