nvptx: Add support for subword compare-and-swap

This adds support for __sync_val_compare_and_swap and
__sync_bool_compare_and_swap for 1-byte and 2-byte long
values, which are not natively supported on nvptx.

Build and reg-tested on nvptx.
Build and reg-tested libgomp on x86_64 with nvptx accelerator.

2020-07-16  Kwok Cheung Yeung  <kcy@codesourcery.com>

	libgcc/
	* config/nvptx/atomic.c: New.
	* config/nvptx/t-nvptx (LIB2ADD): Add atomic.c.

	gcc/testsuite/
	* gcc.target/nvptx/ia64-sync-5.c: New.

	libgomp/
	* testsuite/libgomp.c-c++-common/reduction-16.c: New.