cmd/go: force LANG=C when looking for compiler version

Tested by installing the gcc-locales package and running
        LANG=de_DE.utf8 go build hello.go
    Without this change, that fails, as described at
    https://gcc.gnu.org/PR84765.
    
    Reviewed-on: https://go-review.googlesource.com/100737

From-SVN: r258565

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-ce28919112dbb234366816ab39ce060ad45e8ca9
+e4464efc767b8dee4f4c18ffaf6c891f7b9deee7
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/go/cmd/go/internal/work/buildid.go

@@ -234,7 +234,18 @@ func (b *Builder) gccgoToolID(name, language string) (string, error) {
 	// compile an empty file on standard input.
 	cmdline := str.StringList(cfg.BuildToolexec, name, "-###", "-x", language, "-c", "-")
 	cmd := exec.Command(cmdline[0], cmdline[1:]...)
-	cmd.Env = base.EnvForDir(cmd.Dir, os.Environ())
+
+	// Strip any LANG or LC_ environment variables, and force
+	// LANG=C, so that we get the untranslated output.
+	var env []string
+	for _, e := range os.Environ() {
+		if !strings.HasPrefix(e, "LANG=") && !strings.HasPrefix(e, "LC_") {
+			env = append(env, e)
+		}
+	}
+	env = append(env, "LANG=C")
+
+	cmd.Env = base.EnvForDir(cmd.Dir, env)
 	out, err := cmd.CombinedOutput()
 	if err != nil {
 		return "", fmt.Errorf("%s: %v; output: %q", name, err, out)