fix RT 23810: eval and tied methods

fix RT 23810: eval and tied methods

Something like the following ended up corrupted:
    sub FETCH { eval 'BEGIN{syntax err}' }
The croak on error popped back the context stack etc to the EVAL pushed by
entereval, but the corresponding JUMPENV_PUSH(3) unwound all the way to the
outer perl_run, losing all the mg_get() related parts of the C stack.

It turns out that the run-time parts of pp_entereval were protected with
a new JUMPENV level, but the compile-time parts weren't. Add this.