Do not try to use cleaner if no unsafe

Motivation:

If unsafe is unavailable, we can not use the cleaner anyway. If we try
to set it up, we get an annoying log message about unsafe being
unavailable (when debug logging is enabled). We know this will fail, so
we should not even bother and avoid the log message.

Modifications:

This commit adds a guard against setting up the cleaner if it is not
going to be available because unsafe is unavailable.

Result:

We do not try to set up the cleaner if unsafe is unavailable, and we do
not get an annoying log message.

common/src/main/java/io/netty/util/internal/PlatformDependent.java

@@ -175,7 +175,7 @@ public final class PlatformDependent {
 
         MAYBE_SUPER_USER = maybeSuperUser0();
 
-        if (!isAndroid()) {
+        if (!isAndroid() && hasUnsafe()) {
             // only direct to method if we are not running on android.
             // See https://github.com/netty/netty/issues/2604
             if (javaVersion() >= 9) {