cmd/go: gccgo: consistent results, fix vendoring

    
    Pass the -fdebug-prefix-map and -gno-record-gcc-switches compiler
    options to gccgo to generate consistent results.
    
    Fix the vendoring code to look for /vendor/, not just /vendor, to
    avoid being confused by something like vendor/vendor.org.
    
    Tested by the cmd/go tests in a followup CL.
    
    Reviewed-on: https://go-review.googlesource.com/45695

From-SVN: r249198