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 no longer works (but omp_cgroup_mem_alloc still does).

libgomp/ChangeLog:

	* allocator.c (MEMSPACE_VALIDATE): New macro.
	(omp_init_allocator): Use MEMSPACE_VALIDATE.
	(omp_aligned_alloc): Use OMP_LOW_LAT_MEM_ALLOC_INVALID.
	(omp_aligned_calloc): Likewise.
	(omp_realloc): Likewise.
	* config/nvptx/allocator.c (nvptx_memspace_validate): New function.
	(MEMSPACE_VALIDATE): New macro.
	(OMP_LOW_LAT_MEM_ALLOC_INVALID): New define.
	* libgomp.texi: Document low-latency implementation details.
	* testsuite/libgomp.c/omp_alloc-1.c (main): Add gnu_lowlat.
	* testsuite/libgomp.c/omp_alloc-2.c (main): Add gnu_lowlat.
	* testsuite/libgomp.c/omp_alloc-3.c (main): Add gnu_lowlat.
	* testsuite/libgomp.c/omp_alloc-4.c (main): Add access trait.
	* testsuite/libgomp.c/omp_alloc-5.c (main): Add gnu_lowlat.
	* testsuite/libgomp.c/omp_alloc-6.c (main): Add access trait.
	* testsuite/libgomp.c/omp_alloc-traits.c: New test.