Math builtin definition tweaks.

There were missed optimizations when the system headers didn't have attributes
in place, specifically:

- Add copysign, exp2, log2, nearbyint, rint and trunc to the list.
  These are functions that get inlined by LLVM's optimizer, but only when they
  have the right attributes.
- Mark copysign, fabs, fmax, fmin and trunc const unconditionally.
  Previously these were only const with -fno-math-errno, but they never set
  errno per POSIX.

For ceil/floor/nearbyint/round I'm not aware of any implementation that sets
errno, but POSIX says it may signal overflow so I left them alone for now.

llvm-svn: 162375