Fix a logic bug in the handling of -fmath-errno in the driver. We would
imply -fno-math-errno if the user passed -fno-fast-math OR -ffast-math,
regardless of in which order and regardless of the tool chain default.

I've fixed this to follow the logic:

1) If the last dominating flag is -fno-math-errno, -ffast-math, or
   -Ofast, then do not use math-errno.
2) If the last dominating flag is an explicit -fmath-errno, do use
   math-errno.
3) Otherwise, use the toolchain default.

This, for example, allows the flag sequence
'-ffast-math ... -fno-fast-math' with no mention of '-fmath-errno' or
'-fno-math-errno' to preserve the toolchain default. Most notably, this
should prevent users trying to disable fast-math optimizations on Darwin
and BSD platforms from simultaneously enabling (pointless) -fmath-errno.

I've enhanced the tests (after more reorganization) to cover this and
other weird permutations of flags and targets.