This patch adds support for allocating low-latency ".shared" memory on
NVPTX GPU device, via the omp_low_lat_mem_space and omp_alloc.  The memory
can be allocated, reallocated, and freed using a basic but fast algorithm,
is thread safe and the size of the low-latency heap can be configured using
the GOMP_NVPTX_LOWLAT_POOL environment variable.

The use of the PTX dynamic_smem_size feature means that low-latency allocator
will not work with the PTX 3.1 multilib.

For now, the omp_low_lat_mem_alloc allocator also works, but that will change
when I implement the access traits.

libgomp/ChangeLog:

	* allocator.c (MEMSPACE_ALLOC): New macro.
	(MEMSPACE_CALLOC): New macro.
	(MEMSPACE_REALLOC): New macro.
	(MEMSPACE_FREE): New macro.
	(predefined_alloc_mapping): New array.  Add _Static_assert to match.
	(ARRAY_SIZE): New macro.
	(omp_aligned_alloc): Use MEMSPACE_ALLOC.
	Implement fall-backs for predefined allocators.  Simplify existing
	fall-backs.
	(omp_free): Use MEMSPACE_FREE.
	(omp_calloc): Use MEMSPACE_CALLOC. Implement fall-backs for
	predefined allocators.  Simplify existing fall-backs.
	(omp_realloc): Use MEMSPACE_REALLOC, MEMSPACE_ALLOC, and MEMSPACE_FREE.
	Implement fall-backs for predefined allocators.  Simplify existing
	fall-backs.
	* config/nvptx/team.c (__nvptx_lowlat_pool): New asm variable.
	(__nvptx_lowlat_init): New prototype.
	(gomp_nvptx_main): Call __nvptx_lowlat_init.
	* libgomp.texi: Update memory space table.
	* plugin/plugin-nvptx.c (lowlat_pool_size): New variable.
	(GOMP_OFFLOAD_init_device): Read the GOMP_NVPTX_LOWLAT_POOL envvar.
	(GOMP_OFFLOAD_run): Apply lowlat_pool_size.
	* basic-allocator.c: New file.
	* config/nvptx/allocator.c: New file.
	* testsuite/libgomp.c/omp_alloc-1.c: New test.
	* testsuite/libgomp.c/omp_alloc-2.c: New test.
	* testsuite/libgomp.c/omp_alloc-3.c: New test.
	* testsuite/libgomp.c/omp_alloc-4.c: New test.
	* testsuite/libgomp.c/omp_alloc-5.c: New test.
	* testsuite/libgomp.c/omp_alloc-6.c: New test.

Co-authored-by: Kwok Cheung Yeung <kcy@codesourcery.com>
Co-Authored-By: Thomas Schwinge <thomas@codesourcery.com>