cmd/go: allow buildmode=c-shared for android/arm64.

Also, enable test misc/cgo/testcshared for android/arm64.
c/17245 and c/17246 provide the missing pieces for making
this test work.

"androidtest.bash" now passes on a Nexus 9 (volantis)
device running Android build "LMY48T".

Change-Id: Icb9fd2d17d97e0f04cb18d0cd91640c80fbd3fb4
Reviewed-on: https://go-review.googlesource.com/17333Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/dist/test.go

@@ -375,8 +375,8 @@ func (t *tester) registerTests() {
 			break
 		}
 
-		// Darwin ARM64 fails with internal linking.
-		if t.goos == "darwin" && t.goarch == "arm64" {
+		// Darwin/Android ARM64 fails with internal linking.
+		if (t.goos == "darwin" || t.goos == "android") && t.goarch == "arm64" {
 			break
 		}
 
@@ -618,7 +618,7 @@ func (t *tester) supportedBuildmode(mode string) bool {
 		switch pair {
 		case "linux-386", "linux-amd64", "linux-arm", "linux-arm64",
 			"darwin-amd64",
-			"android-arm", "android-386":
+			"android-arm", "android-arm64", "android-386":
 			return true
 		}
 		return false

src/cmd/go/build.go

@@ -351,7 +351,7 @@ func buildModeInit() {
 		} else {
 			switch platform {
 			case "linux/amd64", "linux/arm", "linux/arm64", "linux/386",
-				"android/amd64", "android/arm", "android/386":
+				"android/amd64", "android/arm", "android/arm64", "android/386":
 				codegenArg = "-shared"
 			case "darwin/amd64":
 			default: