[PowerPC] Floating-point exception trap followup

• Fix incorrect operation on 32-bit caused by incorrectly-sized storage for a temporary FPSCR.
• Fix several whitespace problems.
• Don't try to enable VSX during cleanup_fpscr().

Reviewed by: alfredo, jhibbits (earlier version)
Sponsored by: Tag1 Consulting, Inc.
Differential Revision: https://reviews.freebsd.org/D27453