powerpc: Fix powerpcspe WRT FPSCR

Summary:
Since powerpcspe doesn't have a traditional FPU, there's no FPSCR, and
no FPRs. Attempting to use them triggers an illegal instruction trap.
Fix this unconditional cleanup of FPSCR by conditionalizing it on the
FPU being used in the outgoing thread.

Reviewed By: bdragon
Differential Revision: https://reviews.freebsd.org/D29452