compiler: avoid negative zero in float constants

compiler: avoid negative zero in float constants

    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/91915

From-SVN: r257390