Support OpenMP 5.1's syntax for OMP_ALLOCATOR as well,
which permits besides predefined allocators also
predefined memspaces optionally followed by traits.

Additionally, this commit adds the previously lacking
documentation for OMP_ALLOCATOR, OMP_AFFINITY_FORMAT
and OMP_DISPLAY_AFFINITY.

libgomp/ChangeLog:

	* env.c (gomp_def_allocator_envvar): New var.
	(parse_allocator): Handle OpenMP 5.1 syntax.
	(cleanup_env): New.
	(omp_display_env): Output gomp_def_allocator_envvar
	for an allocator with traits.
	* libgomp.texi (OMP_ALLOCATOR, OMP_AFFINITY_FORMAT,
	OMP_DISPLAY_AFFINITY): New.
	* testsuite/libgomp.c/allocator-1.c: New test.
	* testsuite/libgomp.c/allocator-2.c: New test.
	* testsuite/libgomp.c/allocator-3.c: New test.
	* testsuite/libgomp.c/allocator-4.c: New test.
	* testsuite/libgomp.c/allocator-5.c: New test.
	* testsuite/libgomp.c/allocator-6.c: New test.