cmd/go: add -shared to darwin/arm{,64} default build mode

Buildmode c-archive now supports position independent code for
darwin/arm (in addition to darwin/arm64). Make PIC (-shared) the
default for both platforms in the default buildmode.

Without this change, gomobile will go install the standard library
into its separate package directory without PIC support.

Also add -shared to darwin/arm64 in buildmode c-archive, for
symmetry (darwin/arm64 always generates position independent code).

Fixes #15519

Change-Id: If27d2cbea8f40982e14df25da2703cbba572b5c6
Reviewed-on: https://go-review.googlesource.com/22920Reviewed-by: David Crawshaw <crawshaw@golang.org>
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

misc/cgo/testcarchive/carchive_test.go

@@ -84,7 +84,7 @@ func init() {
 		cc = append(cc, []string{"-framework", "CoreFoundation", "-framework", "Foundation"}...)
 	}
 	libgodir = GOOS + "_" + GOARCH
-	if GOOS == "darwin" && GOARCH == "arm" {
+	if GOOS == "darwin" && (GOARCH == "arm" || GOARCH == "arm64") {
 		libgodir = GOOS + "_" + GOARCH + "_shared"
 	}
 	cc = append(cc, "-I", filepath.Join("pkg", libgodir))

src/cmd/go/build.go

@@ -335,7 +335,7 @@ func buildModeInit() {
 			return p
 		}
 		switch platform {
-		case "darwin/arm":
+		case "darwin/arm", "darwin/arm64":
 			codegenArg = "-shared"
 		default:
 		}
@@ -361,6 +361,9 @@ func buildModeInit() {
 		case "android/arm", "android/arm64", "android/amd64", "android/386":
 			codegenArg = "-shared"
 			ldBuildmode = "pie"
+		case "darwin/arm", "darwin/arm64":
+			codegenArg = "-shared"
+			fallthrough
 		default:
 			ldBuildmode = "exe"
 		}