ls: support --time=creation to show/sort birth time

ls: support --time=creation to show/sort birth time

* src/ls.c (usage): Reorganize help for --time,
and add description for --time=birth.
(do_statx): Store btime in mtime if available.
(get_stat_btime): A new function to read the creation time
from the appropriate stat structure member.
(cmp_btime): A new function to compare birth time.
(print_long_format): Output '?' when birth time unavailable.
* doc/coreutils.texi: Document --time={birth,creation}.
* tests/local.mk: Reference the new test.
* tests/ls/birthtime.sh: Add a new test.
* NEWS: Mention the new feature.