Various minor cleanups and bug fixes to the FP software completion code:
- Use __CTASSERT() instead of rolling our own compile-time assertion
  using cpp.
- Use __BIT() &c instead of rolling our own.
- Improve some comments.
- Define a default FP_C and FPCR value that is self-consistent, and
  initialize it properly at process creation time.
- Fix signal information when the trap shadow cannot be resolved.
- Use defined constants rather than magic numbers for the exception
  summary bits.
- Add a machdep sysctl to enable FP software-completion debugging.

(thorpej)