nptl: Move sem_close, sem_open into libc

The symbols were moved using move-symbol-to-libc.py.

Both functions are moved at the same time because they depend
on internal functions in sysdeps/pthread/sem_routines.c, which
are moved in this commit as well.  Additional hidden prototypes
are required to avoid check-localplt failures.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>