rt: Move shm_open into libc

rt: Move shm_open into libc

This function has no dependency on libpthread, so the move is also
applied to Hurd.

To avoid localplt failures, use __open64_nocancel instead of
pthread_setcancelstate and open.

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

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