libc: Clean up the Symbol.maps a bit.

libc: Clean up the Symbol.maps a bit.

* Remove sysv_cond_wait and sysv_cond_signal because these functions
  were always static.

* Remove tre_have_approx because this function is only available with
  -DTRE_APPROX which we never used, nor build with right away.

* Remove pthread_init since there was never a pthread_init in libc.
  Seems to be just a typo.

* Remove mexitcount which was kernel only.

* Add a comment to a symbol that will only show up when libc is built
  with a debug option.