llvm-go: Set $GCCGO instead of putting a gccgo executable on $PATH.

[pcc]

Summary

Now that llgo ships its own go command we can rely on it having support for $GCCGO.

[axw]

LGTM

llvm/trunk/tools/llvm-go/llvm-go.go

for _, p := range packages {
     err = os.Symlink(filepath.Join(srcdir, p.llvmpath), path)
     if err != nil {
       panic(err.Error())
     }
   }
 
   newpath := os.Getenv("PATH")
 
-  if llgo != "" {
-    bindir := filepath.Join(tmpgopath, "bin")
-
-    err = os.MkdirAll(bindir, os.ModePerm)
-    if err != nil {
-      panic(err.Error())
-    }
-
-    err = os.Symlink(llgo, filepath.Join(bindir, "gccgo"))
-    if err != nil {
-      panic(err.Error())
-    }
-
-    newpathlist := []string{bindir}
-    newpathlist = append(newpathlist, filepath.SplitList(newpath)...)
-    newpath = strings.Join(newpathlist, string(filepath.ListSeparator))
-
-    args = append([]string{args[0], "-compiler", "gccgo"}, args[1:]...)
-  }
-
   newgopathlist := []string{tmpgopath}
   newgopathlist = append(newgopathlist, filepath.SplitList(os.Getenv("GOPATH"))...)
   newgopath := strings.Join(newgopathlist, string(filepath.ListSeparator))
 
   flags := llvmFlags()
 
   newenv := []string{
     "CC=" + cc,
     "CXX=" + cxx,
     "CGO_CPPFLAGS=" + flags.cpp + " " + cppflags,
     "CGO_CXXFLAGS=" + flags.cxx + " " + cxxflags,
     "CGO_LDFLAGS=" + flags.ld + " " + ldflags,
     "GOPATH=" + newgopath,
     "PATH=" + newpath,
   }
+  if llgo != "" {
+    newenv = append(newenv, "GCCGO=" + llgo)
+  }
+
   for _, v := range os.Environ() {
     if !strings.HasPrefix(v, "CC=") &&
       !strings.HasPrefix(v, "CXX=") &&
       !strings.HasPrefix(v, "CGO_CPPFLAGS=") &&
       !strings.HasPrefix(v, "CGO_CXXFLAGS=") &&
       !strings.HasPrefix(v, "CGO_LDFLAGS=") &&
+      !strings.HasPrefix(v, "GCCGO=") &&
       !strings.HasPrefix(v, "GOPATH=") &&
       !strings.HasPrefix(v, "PATH=") {
       newenv = append(newenv, v)
     }
   }
 
   gocmdpath, err := exec.LookPath(gocmd)
   if err != nil {