[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

(cherry picked from commit 4f9ed3156c3aff08629d37c8a89ed5ba525b01c9)