cmd/go: pass gccgoflags at the end of gccgo command line, warn if user passes...

cmd/go: pass gccgoflags at the end of gccgo command line, warn if user passes the wrong toolchain options

R=iant
CC=golang-dev
https://golang.org/cl/6940082

src/cmd/go/build.go

@@ -191,6 +191,21 @@ func runBuild(cmd *Command, args []string) {
 		*buildO += exeSuffix
 	}
 
+	// sanity check some often mis-used options
+	switch buildContext.Compiler {
+	case "gccgo":
+		if len(buildGcflags) != 0 {
+			fmt.Println("go build: when using gccgo toolchain, please pass compiler flags using -gccgoflags, not -gcflags")
+		}
+		if len(buildLdflags) != 0 {
+			fmt.Println("go build: when using gccgo toolchain, please pass linker flags using -gccgoflags, not -ldflags")
+		}
+	case "gc":
+		if len(buildGccgoflags) != 0 {
+			fmt.Println("go build: when using gc toolchain, please pass compile flags using -gcflags, and linker flags using -ldflags")
+		}
+	}
+
 	if *buildO != "" {
 		if len(pkgs) > 1 {
 			fatalf("go build: cannot use -o with multiple packages")
@@ -1451,7 +1466,7 @@ func (tools gccgcToolchain) ld(b *builder, p *Package, out string, allactions []
 	if usesCgo && goos == "linux" {
 		ldflags = append(ldflags, "-Wl,-E")
 	}
-	return b.run(".", p.ImportPath, "gccgo", "-o", out, buildGccgoflags, ofiles, "-Wl,-(", ldflags, "-Wl,-)")
+	return b.run(".", p.ImportPath, "gccgo", "-o", out, ofiles, "-Wl,-(", ldflags, "-Wl,-)", buildGccgoflags)
 }
 
 func (gccgcToolchain) cc(b *builder, p *Package, objdir, ofile, cfile string) error {