cmd/gomobile: update comments to explain why x86_64 is used for Arm64

Android NDK provides universal binaries both for Intel and Arm
architectures without changing the path from r24. Thus, we should keep
using x86_64 as a part of path. This change updates the comment to
explain the situation.

Change-Id: I1368745f5d7d2b018fb88a0cc83e27846402bbd9
Reviewed-on: https://go-review.googlesource.com/c/mobile/+/746140
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Mark Freeman <markfreeman@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

Author: hajimehoshi

cmd/gomobile/env.go

@@ -528,9 +528,13 @@ func archNDK() string {
 		case "amd64":
 			arch = "x86_64"
 		case "arm64":
-			// Android NDK does not contain arm64 toolchains (until and
-			// including NDK 23), use use x86_64 instead. See:
-			// https://github.com/android/ndk/issues/1299
+			// Until NDK 23, Android NDK does not contain Arm64 toolchains.
+			// From NDK 24, Android NDK supports Arm64 as universal binaries without changing the path.
+			// Use "x86_64" as a part of the path.
+			// See also:
+			// * https://developer.android.com/ndk/guides/other_build_systems
+			// * https://github.com/android/ndk/wiki/Changelog-r24
+			// * https://github.com/android/ndk/issues/1299
 			if runtime.GOOS == "darwin" {
 				arch = "x86_64"
 				break