[nvptx] Handle memmodel for atomic ops

[nvptx] Handle memmodel for atomic ops

The atomic ops in nvptx.md have memmodel arguments, which are currently
ignored.

Handle these, fixing test-case fails libgomp.c-c++-common/reduction-{5,6}.c
on volta.

Tested libgomp on x86_64-linux with nvptx accelerator.

gcc/ChangeLog:

2021-05-17  Tom de Vries  <tdevries@suse.de>

	PR target/100497
	* config/nvptx/nvptx-protos.h (nvptx_output_atomic_insn): Declare
	* config/nvptx/nvptx.c (nvptx_output_barrier)
	(nvptx_output_atomic_insn): New function.
	(nvptx_print_operand): Add support for 'B'.
	* config/nvptx/nvptx.md: Use nvptx_output_atomic_insn for atomic
	insns.