[MIPS] Maintain si_code field properly for FP exceptions

The appended patch adds code to update siginfo_t's si_code field. It
fixes e.g. a floating point overflow regression in the SBCL testsuite.

Signed-off-By: Thiemo Seufer <ths@linux-mips.org>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>