Include <stddef.h>, not <stdlib.h> and <sys/types.h>.

Include <stddef.h>, not <stdlib.h> and <sys/types.h>.
(LONG_MAX_32_BITS): Remove.
Include <inttypes.h> and <stdint.h> if available.
(alignof, UNALIGNEDP): New macro, portable to all C89 hosts.
(__memrchr): Don't assume unsigned long int is either 4 or 8 bytes;
let it be any number of bytes greater than or equal to 4.