openmp, nvptx: low-lat memory access traits

openmp, nvptx: low-lat memory access traits

The NVPTX low latency memory is not accessible outside the team that allocates
it, and therefore should be unavailable for allocators with the access trait
"all".  This change means that the omp_low_lat_mem_alloc predefined
allocator now implicitly implies the "pteam" trait.

	Backport of a patch posted at
	https://gcc.gnu.org/pipermail/gcc-patches/2022-January/589355.html

libgomp/ChangeLog:

	* allocator.c (MEMSPACE_VALIDATE): New macro.
	(omp_aligned_alloc): Use MEMSPACE_VALIDATE.
	(omp_aligned_calloc): Likewise.
	(omp_realloc): Likewise.
	* config/nvptx/allocator.c (nvptx_memspace_validate): New function.
	(MEMSPACE_VALIDATE): New macro.
	* testsuite/libgomp.c/allocators-4.c (main): Add access trait.
	* testsuite/libgomp.c/allocators-6.c (main): Add access trait.
	* testsuite/libgomp.c/allocators-7.c: New test.