Create an "interpreted" version of Continuation.enterSpecial

To properly fix JDK-8288949 (in a way that avoids the possibility of another thread, running concurrently to an interp_only_mode thread resolving the call to Continuation.enter) we should probably have an "interpreted" version of enterSpecial.

There is no need for a special "deoptimization" of such frames, because there's only one call to Java (and one safepoint) from enterSpecial. There's nothing to be done after we return to it.