cmd/go: -buildmode=c-shared should work on darwin/386

* Enable c-shared buildmode on darwin/386
* dyld does not support text relocation on i386. Add -read_only_relocs suppress flag to linker

Fixes #13904

Change-Id: I9adbd20d3f36ce9bbccf1bffb746b391780d088f
Reviewed-on: https://go-review.googlesource.com/18500Reviewed-by: David Crawshaw <crawshaw@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

misc/cgo/testcshared/test.bash

@@ -27,7 +27,7 @@ fi
 # Directory where cgo headers and outputs will be installed.
 # The installation directory format varies depending on the platform.
 installdir=pkg/${goos}_${goarch}_testcshared_shared
-if [ "${goos}/${goarch}" == "darwin/amd64" ]; then
+if [ "${goos}" == "darwin" ]; then
 	installdir=pkg/${goos}_${goarch}_testcshared
 fi
 

src/cmd/dist/test.go

@@ -656,7 +656,7 @@ func (t *tester) supportedBuildmode(mode string) bool {
 	case "c-shared":
 		switch pair {
 		case "linux-386", "linux-amd64", "linux-arm", "linux-arm64",
-			"darwin-amd64",
+			"darwin-amd64", "darwin-386",
 			"android-arm", "android-arm64", "android-386":
 			return true
 		}

src/cmd/go/build.go

@@ -354,7 +354,7 @@ func buildModeInit() {
 			case "linux/amd64", "linux/arm", "linux/arm64", "linux/386",
 				"android/amd64", "android/arm", "android/arm64", "android/386":
 				codegenArg = "-shared"
-			case "darwin/amd64":
+			case "darwin/amd64", "darwin/386":
 			default:
 				fatalf("-buildmode=c-shared not supported on %s\n", platform)
 			}

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

@@ -1068,7 +1068,7 @@ func hostlink() {
 		argv = append(argv, "-pie")
 	case BuildmodeCShared:
 		if HEADTYPE == obj.Hdarwin {
-			argv = append(argv, "-dynamiclib")
+			argv = append(argv, "-dynamiclib", "-Wl,-read_only_relocs,suppress")
 		} else {
 			// ELF.
 			argv = append(argv, "-Wl,-Bsymbolic")