libgo: eliminate a lot of duplication in Makefile.am

    
    I read through the GNU make manual.  I knew there had to be a way to do it.
    
    Remove the special netgo library.  The essential feature--using the Go
    DNS resolver--is now available by setting GODEBUG=netdns=go.
    
    Reviewed-on: https://go-review.googlesource.com/32333

From-SVN: r241687