[MIPS] Signal cleanup

    
Move function prototypes to asm/signal.h to detect trivial errors and
add some __user tags to get rid of sparse warnings.  Generated code
should not be changed.
    
Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>