cmd/go: allow -buildmode=pie for android/amd64.

For golang/go#10743 golang/go#10807

Change-Id: I35dedb52e2b47fe7ffc655d01f22ac05fe830bde
Reviewed-on: https://go-review.googlesource.com/16396
Reviewed-by: David Crawshaw <crawshaw@golang.org>

build.go

@@ -361,7 +361,7 @@ func buildModeInit() {
 		ldBuildmode = "c-shared"
 	case "default":
 		switch platform {
-		case "android/arm":
+		case "android/arm", "android/amd64":
 			codegenArg = "-shared"
 			ldBuildmode = "pie"
 		default:
@@ -375,7 +375,7 @@ func buildModeInit() {
 			fatalf("-buildmode=pie not supported by gccgo")
 		} else {
 			switch platform {
-			case "android/arm", "linux/amd64":
+			case "android/arm", "linux/amd64", "android/amd64":
 				codegenArg = "-shared"
 			default:
 				fatalf("-buildmode=pie not supported on %s\n", platform)