libgomp, nvptx: low-latency memory allocator

libgomp, nvptx: low-latency memory allocator

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 the minimum version
requirement is now bumped to 4.1 (still old at this point).

libgomp/ChangeLog:

	* allocator.c (MEMSPACE_ALLOC): New macro.
	(MEMSPACE_CALLOC): New macro.
	(MEMSPACE_REALLOC): New macro.
	(MEMSPACE_FREE): New macro.
	(dynamic_smem_size): New constants.
	(omp_alloc): Use MEMSPACE_ALLOC.
	Implement fall-backs for predefined allocators.
	(omp_free): Use MEMSPACE_FREE.
	(omp_calloc): Use MEMSPACE_CALLOC.
	Implement fall-backs for predefined allocators.
	(omp_realloc): Use MEMSPACE_REALLOC.
	Implement fall-backs for predefined allocators.
	* config/nvptx/team.c (__nvptx_lowlat_heap_root): New variable.
	(__nvptx_lowlat_pool): New asm varaible.
	(gomp_nvptx_main): Initialize the low-latency heap.
	* 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.
	* config/nvptx/allocator.c: New file.
	* testsuite/libgomp.c/allocators-1.c: New test.
	* testsuite/libgomp.c/allocators-2.c: New test.
	* testsuite/libgomp.c/allocators-3.c: New test.
	* testsuite/libgomp.c/allocators-4.c: New test.
	* testsuite/libgomp.c/allocators-5.c: New test.
	* testsuite/libgomp.c/allocators-6.c: New test.