powerpcspe: Implement SPE exception handling

The Signal Processing Engine (SPE) found in Freescale e500 cores (and
others) offloads IEEE-754 compliance (NaN, Inf handling, overflow,
underflow) to software, most likely as a means of simplifying the APU
silicon. Some software, like AbiWord, needs full IEEE-754 compliance,
including NaN handling. Implement the necessary bits to enable it.

Differential Revision: https://reviews.freebsd.org/D17446