We were using special compilation flags for the math package, but we
    weren't using them when testing.  That meant that our tests were not
    checking the real code we were providing.  Fix that.
    
    Fixing that revealed that we were not using a good set of flags, or at
    least were not using flags that let the tests pass.  Adjust the flags
    to stop using -funsafe-math-optimizations on x86.  Instead always use
    -ffp-contract=off -fno-math-errno -fno-trapping-math for all targets.
    
    Fixes golang/go#23647
    
    Reviewed-on: https://go-review.googlesource.com/91355

From-SVN: r257312