Change Continuations::enabled to return true on ports with continuations implemented. This will turn on the generation of the thaw and return barrier stubs and also change code generation to emit post-call nops. This code currently requires --enable-preview but needs to be changed to be enabled by default so that any issues can be shaken out as early as possible in JDK 21, and in advance of deciding whether or not to propose virtual threads as a permanent feature. At this point, we believe the performance regression related to the interaction between the post-call nops and inlining has been addressed JDK-8300002.