[nvptx] Handle pre-sm_7x shared atomic store using atomic exchange

[nvptx] Handle pre-sm_7x shared atomic store using atomic exchange

The ptx isa specifies (for pre-sm_7x) that atomic operations on shared memory
locations do not guarantee atomicity with respect to normal store instructions
to the same address.

This can be fixed by:
- inserting barriers between normal stores and atomic operations to a common
  address
- using atom.exch to store to locations accessed by other atomic operations.

It's not clearly spelled out which barriers are needed, and a barrier seem more
expensive than atomic exchange.

Implement the pre-sm_7x shared atomic store using atomic exchange.

That includes stores using generic addressing, since those may also point to
shared memory.

Tested on x86-64 with nvptx accelerator.

gcc/ChangeLog:

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

	* config/nvptx/nvptx-protos.h (nvptx_mem_maybe_shared_p): Declare.
	* config/nvptx/nvptx.cc (nvptx_mem_data_area): New static function.
	(nvptx_mem_maybe_shared_p): New function.
	* config/nvptx/nvptx.md (define_expand "atomic_store<mode>"): New
	define_expand.

gcc/testsuite/ChangeLog:

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

	* gcc.target/nvptx/atomic-store-1.c: New test.
	* gcc.target/nvptx/atomic-store-3.c: New test.
	* gcc.target/nvptx/stack-atomics-run.c: Update.