cmd/go: pass down testing gccgo in TestScript

cmd/go: pass down testing gccgo in TestScript

    This permits TestScript to work when gccgo is not installed.
    Previous testing was using a previously installed gccgo, not the newly
    built one.

    This revealed that the testing of whether an internal package is
    permitted was incorrect for standard library packages, since the
    uninstalled gccgo can see internal packages in the uninstalled libgo.
    Fix the internal package tests.

    This permitted removing a couple of gccgo-specific changes in the
    testsuite.

    Reviewed-on: https://go-review.googlesource.com/137255

From-SVN: r264570