MIPS: Dumb MSA FP exception handler

MIPS: Dumb MSA FP exception handler

This patch adds a simple handler for MSA FP exceptions which delivers a
SIGFPE to the running task. In the future it should probably be extended
to re-execute the instruction with the MSACSR.NX bit set in order to
generate results for any elements which did not cause an exception
before delivering the SIGFPE signal.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/6432/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>