compiler, runtime: panic on uncomparable map key, even if map is empty

compiler, runtime: panic on uncomparable map key, even if map is empty

    This ports https://golang.org/cl/155918 from the master repo.

        runtime: panic on uncomparable map key, even if map is empty

        Reorg map flags a bit so we don't need any extra space for the extra flag.

    This is a pre-req for updating libgo to the Go 1.12beta2 release.

    Reviewed-on: https://go-review.googlesource.com/c/157858

From-SVN: r267950