cmd/link: support C-shared buildmode on macOS/ARM64

It just works, after the plugin work.

Updates #38485.

Change-Id: I55aa11b380a33a729fccb731b77f48bc7d0dea2e
Reviewed-on: https://go-review.googlesource.com/c/go/+/259443
Trust: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>

src/cmd/dist/test.go

@@ -992,7 +992,7 @@ func (t *tester) supportedBuildmode(mode string) bool {
 	case "c-shared":
 		switch pair {
 		case "linux-386", "linux-amd64", "linux-arm", "linux-arm64", "linux-ppc64le", "linux-s390x",
-			"darwin-amd64",
+			"darwin-amd64", "darwin-arm64",
 			"freebsd-amd64",
 			"android-arm", "android-arm64", "android-386",
 			"windows-amd64", "windows-386":

src/cmd/internal/sys/supported.go

@@ -69,7 +69,7 @@ func BuildModeSupported(compiler, buildmode, goos, goarch string) bool {
 		case "linux/amd64", "linux/arm", "linux/arm64", "linux/386", "linux/ppc64le", "linux/s390x",
 			"android/amd64", "android/arm", "android/arm64", "android/386",
 			"freebsd/amd64",
-			"darwin/amd64",
+			"darwin/amd64", "darwin/arm64",
 			"windows/amd64", "windows/386":
 			return true
 		}

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

@@ -1329,9 +1329,6 @@ func (ctxt *Link) hostlink() {
 	case BuildModeCShared:
 		if ctxt.HeadType == objabi.Hdarwin {
 			argv = append(argv, "-dynamiclib")
-			if ctxt.Arch.Family != sys.AMD64 {
-				argv = append(argv, "-Wl,-read_only_relocs,suppress")
-			}
 		} else {
 			// ELF.
 			argv = append(argv, "-Wl,-Bsymbolic")