MIPS: Support sending SIG_SYS to 32bit userspace from 64bit kernel

MIPS: Support sending SIG_SYS to 32bit userspace from 64bit kernel

The seccomp_bpf self test revealed that a 64bit kernel delivered an
invalid SIG_SYS to a 32bit userspace, because it was falling into the
default of the switch statement. Add a case to handle delivering the
signal.

With this patch, the seccomp_bpf self test now passes the TRAP.handler
case with O32 and N32 userlands.

Signed-off-by: Matt Redfearn <matt.redfearn@imgtec.com>
Acked-by: Kees Cook <keescook@chromium.org>
Cc: Paul Burton <paul.burton@imgtec.com>
Cc: Amanieu d'Antras <amanieu@gmail.com>
Cc: Alex Smith <alex.smith@imgtec.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: linux-mips@linux-mips.org
Cc: IMG-MIPSLinuxKerneldevelopers@imgtec.com
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/12915/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>