Do not avoid splitting if nothing currently unmapped (USE_MUNMAP)

Do not avoid splitting if nothing currently unmapped (USE_MUNMAP)

* allchblk.c (GC_free_bytes, GC_enough_large_bytes_left,
GC_compute_large_free_bytes): Define even for USE_MUNMAP case.
* allchblk.c (INCR_FREE_BYTES, FREE_ASSERT): Always define as
non-empty.
* allchblk.c (GC_compute_large_free_bytes, GC_print_hblkfreelist,
GC_remove_from_fl): Remove check of USE_MUNMAP (same implementation as
if without memory unmapping).
* allchblk.c (GC_compute_large_free_bytes): Remove FIXME.
* allchblk.c (GC_allochblk): Do not avoid splitting if
GC_unmapped_bytes is zero (meaning nothing unmapped); add FIXME to
relax the condition.