all: Add prebuild binaries for arm64

Change-Id: Ia8a0e2cce7a33c50d0064cb67a4e1c669cf8569b
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/419714
Reviewed-by: Michael Stapelberg <stapelberg@google.com>
Reviewed-by: Damien Neil <dneil@google.com>

integration_test.go

@@ -385,7 +385,7 @@ func mustHandleFlags(t *testing.T) {
 		t.Run("BuildRelease", func(t *testing.T) {
 			v := version.String()
 			for _, goos := range []string{"linux", "darwin", "windows"} {
-				for _, goarch := range []string{"386", "amd64"} {
+				for _, goarch := range []string{"386", "amd64", "arm64"} {
 					// Avoid Darwin since 10.15 dropped support for i386.
 					if goos == "darwin" && goarch == "386" {
 						continue