(POINTER, SIZE) [emacs]: Define in terms of

(POINTER, SIZE) [emacs]: Define in terms of
POINTER_TYPE and size_t.
(struct mmap_region) [REL_ALLOC_MMAP]: New structure.
(mmap_regions, mmap_regions_1) [REL_ALLOC_MMAP]: New variables.
(ROUND, MMAP_REGION_STRUCT_SIZE, MMAP_REGION, MMAP_USER_AREA)
[REL_ALLOC_MMAP]: New macros.
(mmap_find, mmap_free, mmap_enlarge, mmap_set_vars)
(mmap_mapped_bytes, r_alloc, r_re_alloc, r_alloc_free)
[REL_ALLOC_MMAP]: New functions.