cleanup dynamic core exec entry point
remove push/pop ebp from dynamic core exec function. add comment about
how GDB is unable to stack trace properly for anything invoked from the
dynamically generated code.

NOTE: I've determined that the reason C++ exceptions don't work properly
sometimes when core=dynamic is *because* the C++ runtime can't trace the
stack, therefore the exception stops before the handler and the runtime
acts as if nobody caught it!