cmd/go: implement -buildmode=plugin for s390x

Simple change to allow plugins for linux on s390x

Change-Id: I5c262ab81aac10d1dcb03381a48e5b9694b7a87a
Reviewed-on: https://go-review.googlesource.com/37451
Run-TryBot: Michael Munday <munday@ca.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/dist/test.go

@@ -785,7 +785,7 @@ func (t *tester) supportedBuildmode(mode string) bool {
 		// linux-arm64 is missing because it causes the external linker
 		// to crash, see https://golang.org/issue/17138
 		switch pair {
-		case "linux-386", "linux-amd64", "linux-arm":
+		case "linux-386", "linux-amd64", "linux-arm", "linux-s390x":
 			return true
 		}
 		return false

src/cmd/go/internal/work/build.go

@@ -347,7 +347,7 @@ func BuildModeInit() {
 			codegenArg = "-fPIC"
 		} else {
 			switch platform {
-			case "linux/amd64", "linux/arm", "linux/arm64", "linux/386",
+			case "linux/amd64", "linux/arm", "linux/arm64", "linux/386", "linux/s390x",
 				"android/amd64", "android/arm", "android/arm64", "android/386":
 			default:
 				base.Fatalf("-buildmode=plugin not supported on %s\n", platform)

src/cmd/link/internal/ld/config.go

@@ -84,7 +84,7 @@ func (mode *BuildMode) Set(s string) error {
 		switch obj.GOOS {
 		case "linux":
 			switch obj.GOARCH {
-			case "386", "amd64", "arm", "arm64":
+			case "386", "amd64", "arm", "arm64", "s390x":
 			default:
 				return badmode()
 			}

src/cmd/link/internal/s390x/asm.go

@@ -52,7 +52,7 @@ func gentext(ctxt *ld.Link) {
 		return
 	}
 	addmoduledata := ctxt.Syms.Lookup("runtime.addmoduledata", 0)
-	if addmoduledata.Type == obj.STEXT {
+	if addmoduledata.Type == obj.STEXT && ld.Buildmode != ld.BuildmodePlugin {
 		// we're linking a module containing the runtime -> no need for
 		// an init function
 		return
@@ -89,7 +89,9 @@ func gentext(ctxt *ld.Link) {
 
 	// undef (for debugging)
 	ld.Adduint32(ctxt, initfunc, 0)
-
+	if ld.Buildmode == ld.BuildmodePlugin {
+		ctxt.Textp = append(ctxt.Textp, addmoduledata)
+	}
 	ctxt.Textp = append(ctxt.Textp, initfunc)
 	initarray_entry := ctxt.Syms.Lookup("go.link.addmoduledatainit", 0)
 	initarray_entry.Attr |= ld.AttrLocal