avoid tls gets when handling interrupts in the vm

avoid tls gets when handling interrupts in the vm

* libguile/__scm.h (SCM_ASYNC_TICK_WITH_CODE): Redefine to take a
  scm_i_thread* as well.  OK to do because it's within a
  BUILDING_LIBGUILE block.

* libguile/vm-engine.c (vm_engine): Cache the scm_i_thread* instead of
  the dynstate, so we can use the thread for ticks.

* libguile/vm-engine.h (VM_HANDLE_INTERRUPTS): Tick with the
  scm_i_thread* local var, to avoid excessive tls calls.

* libguile/vm-i-system.c: Fix dynstate users to use
  current_thread->dynamic_state.