[MIPS] Add support for TIF_RESTORE_SIGMASK for signal32

    
Following the recent implementation of TIF_RESTORE_SIGMASK in
arch/mips/kernel/signal.c, 64-bit kernels with 32-bit user-land
compatibility oops when starting init.  signal32.c needs to be
converted to use TIF_RESTORE_SIGMASK too.
    
Signed-off-by: Martin Michlmayr <tbm@cyrius.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>