cmd: Use correct install tool dir with gccgo

cmd: Use correct install tool dir with gccgo

    When using the go command built from gccgo to build and
    install a go tool, use the value from runtime GCCGOTOOLDIR as
    the install directory.

    This also fixes the output from 'go tool' when used with the
    gccgo-built go command, to only include the go tools and not
    other binaries found in the same directory.

    Reviewed-on: https://go-review.googlesource.com/16516

From-SVN: r230677