Check for negative numbers with very small magnitudes that will round
    to negative zero, and force them to positive zero instead.
    
    This implements the spec clarification in https://golang.org/cl/14727.
    The test is in https://golang.org/cl/91895.
    
    Fixes golang/go#12621
    
    Reviewed-on: https://go-review.googlesource.com/92175

From-SVN: r257415