MIPS: add seccomp syscall

Wires up the new seccomp syscall.

Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>

Conflicts:
	arch/mips/include/uapi/asm/unistd.h
	arch/mips/kernel/scall32-o32.S
	arch/mips/kernel/scall64-64.S
	arch/mips/kernel/scall64-n32.S
	arch/mips/kernel/scall64-o32.S

Change-Id: I7031bdbec7c90292aeb7e255c73cb36e6ec43af2