Make the missing symbols script more useful.

If something's in POSIX, we don't need to show it in the glibc list.

If something's not implemented by glibc, we don't need to show it in the
POSIX list (because it's probably either a macro or obsolete).

Change-Id: Ied0f8d97d3fffb280c22e9cdf6782430d776c02f
Merge "Make the missing symbols script more useful."