powerpc: Fix powerpcspe WRT FPSCR

[jhibbits]

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.

[bdragon]

Makes sense to me.