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