[PowerPC] Fix SPE floating point environment manipulation

Fix multiple problems in the powerpcspe floating point code.

• Endianness handling of the SPEFSCR in fenv.h was completely broken.
• Ensure SPEFSCR synchronization requirements are being met.

The r.d -> __r transformations were written by jhibbits.

Reviewed by: jhibbits
Differential Revision: https://reviews.freebsd.org/D22526