libgomp: pinned memory

libgomp: pinned memory

Implement the OpenMP pinned memory trait on Linux hosts using the mlock
syscall.  Pinned allocations are performed using mmap, not malloc, to ensure
that they can be unpinned safely when freed.

	Backport of a patch posted at
	https://gcc.gnu.org/pipermail/gcc-patches/2022-January/588360.html

libgomp/ChangeLog:
	* allocator.c (MEMSPACE_ALLOC): Add PIN.
	(MEMSPACE_CALLOC): Add PIN.
	(MEMSPACE_REALLOC): Add PIN.
	(MEMSPACE_FREE): Add PIN.
	(xmlock): New function.
	(omp_init_allocator): Don't disallow the pinned trait.
	(omp_aligned_alloc): Add pinning to all MEMSPACE_* calls.
	(omp_aligned_calloc): Likewise.
	(omp_realloc): Likewise.
	(omp_free): Likewise.
	* config/linux/allocator.c: New file.
	* config/nvptx/allocator.c (MEMSPACE_ALLOC): Add PIN.
	(MEMSPACE_CALLOC): Add PIN.
	(MEMSPACE_REALLOC): Add PIN.
	(MEMSPACE_FREE): Add PIN.
	* testsuite/libgomp.c/alloc-pinned-1.c: New test.
	* testsuite/libgomp.c/alloc-pinned-2.c: New test.
	* testsuite/libgomp.c/alloc-pinned-3.c: New test.
	* testsuite/libgomp.c/alloc-pinned-4.c: New test.