less overhead for handling interrupts in the vm

less overhead for handling interrupts in the vm

* libguile/__scm.h (SCM_ASYNC_TICK): Add some branch prediction.
  (SCM_ASYNC_TICK_WITH_CODE): New helper for when BUILDING_LIBGUILE,
  runs code only if we're going to call async_click().
* libguile/vm-engine.h (VM_HANDLE_INTERRUPTS): New helper, uses
  SCM_ASYNC_TICK_WITH_CODE to only save regs if we'll handle an
  interrupt.
* libguile/vm-i-system.c (call, goto/args, return): use
  VM_HANDLE_INTERRUPTS.