cmd/dist, cmd/go: run testshared on arm64

And enable PIE in the go tool.

Change-Id: Ibb60ccfe62518cde6e33080bbc78bfcbecff6a4e
Reviewed-on: https://go-review.googlesource.com/14000Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>

src/cmd/dist/test.go

@@ -592,7 +592,7 @@ func (t *tester) supportedBuildmode(mode string) bool {
 		return false
 	case "shared":
 		switch pair {
-		case "linux-amd64", "linux-arm", "linux-ppc64le":
+		case "linux-amd64", "linux-arm", "linux-arm64", "linux-ppc64le":
 			return true
 		}
 		return false

src/cmd/go/build.go

@@ -375,7 +375,7 @@ func buildModeInit() {
 			fatalf("-buildmode=pie not supported by gccgo")
 		} else {
 			switch platform {
-			case "linux/arm", "android/arm", "linux/amd64", "android/amd64", "linux/ppc64le":
+			case "linux/arm", "android/arm", "linux/amd64", "android/amd64", "linux/arm64", "linux/ppc64le":
 				codegenArg = "-shared"
 			default:
 				fatalf("-buildmode=pie not supported on %s\n", platform)