msun fixes for SPE

Summary:
Fix FPU exception management for powerpcspe. Bits are in a different place from
the standard FPSCR, so we need to handle the shifting differences. Also,
there's no concept of a "software exception" raise, so we need to do exceptional
math to trigger the exception from software.

Reviewed By: alfredo
Differential Revision: https://reviews.freebsd.org/D22824