nvptx, libgomp: Move the low-latency allocator code

nvptx, libgomp: Move the low-latency allocator code

There shouldn't be a functionality change; this is just so AMD can share
the code.

The new basic-allocator.c is designed to be included so it can be used as a
template multiple times and inlined.

libgomp/ChangeLog:

	* config/nvptx/allocator.c (BASIC_ALLOC_PREFIX): New define, and
	include basic-allocator.c.
	(__nvptx_lowlat_heap_root): Remove.
	(heapdesc): Remove.
	(nvptx_memspace_alloc): Move implementation to basic-allocator.c.
	(nvptx_memspace_calloc): Likewise.
	(nvptx_memspace_free): Likewise.
	(nvptx_memspace_realloc): Likewise.
	* config/nvptx/team.c (__nvptx_lowlat_heap_root): Remove.
	(gomp_nvptx_main): Call __nvptx_lowlat_init.
	* basic-allocator.c: New file.