MIPS: compat: Implement is_compat_task.

This is a build fix required after "x86-64: seccomp: fix 32/64 syscall
hole" (commit 5b101740).  MIPS doesn't
have the issue that was fixed for x86-64 by that patch.

This also doesn't solve the N32 issue which is that N32 seccomp processes
will be treated as non-compat processes thus only have access to N64
syscalls.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>