* alloc.c: Do not assume sizeof (size_t) is a multiple of 8

* alloc.c: Do not assume sizeof (size_t) is a multiple of 8

when debugging.
(alignof, XMALLOC_BASE_ALIGNMENT, XMALLOC_HEADER_ALIGNMENT)
(XMALLOC_OVERRUN_SIZE_SIZE): New macros.
(XMALLOC_OVERRUN_CHECK_OVERHEAD, xmalloc_put_size, xmalloc_get_size)
(overrun_check_malloc, overrun_check_realloc, overrun_check_free):
Replace uses of sizeof (size_t) with XMALLOC_OVERRUN_SIZE_SIZE.