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

gcc/go/gofrontend/MERGE

-fe38baff61b9b9426a4f60ff078cf3c8722bf94d
+4ca21c94f00c620bfde2f924e214c78fe2e1ff69
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.