nptl: Move sem_unlink into libc

The symbol was moved using scripts/move-symbol-to-libc.py.

A small adjust to the sem_unlink implementation is necessary to avoid
a check-localplt failure.

A placeholder symbol to keep the GLIBC_2.1.1 version alive in
libpthread is added with this commit.

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