[MIPS] signals: reduce {setup,restore}_sigcontext sizes

This trivial change reduces considerably code size of these
2 functions callers. For instance, here is the figures for
arch/kernel/signal.o objects:

   text    data     bss     dec     hex filename
  11972       0       0   11972    2ec4 arch/mips/kernel/signal.o~old
   5380       0       0    5380    1504 arch/mips/kernel/signal.o~new

Signed-off-by: Franck Bui-Huu <fbuihuu@gmail.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>