compiler: rollback "avoid negative zero in float constants"

compiler: rollback "avoid negative zero in float constants"

    It uses functions that are not available in MPFR 2.4.2, which is the
    current version supported by GCC.

    Original change description:

        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

    Updates golang/go#12621

    Reviewed-on: https://go-review.googlesource.com/92055

From-SVN: r257393