m68k: Really wire up sys_pselect6 and sys_ppoll

We reserved the numbers a long time ago, but never wired them up in the
syscall table as they need TIF_RESTORE_SIGMASK, which we only got last year
in commit cb6831d5d3099e772a510eb3e1ed0760ccffb45e ("m68k: Switch to saner
sigsuspend()")

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Greg Ungerer <gerg@uclinux.org>
Cc: stable@kernel.org