ec_switch can still lose interrupts
ec_switch and thread switching may still lose interrupts
trap interrupt is OK because of r64062
1) postponed job interrupt from MJIT worker
This is trickiest to solve because it also affects thread switching,
not just EC switching
2) pending interrupt is not safe but fixing ec_switch is sufficient because
rb_threadptr_interrupt only targets threads, not EC.
3) timer interrupt is not critical because another interrupt will fire in 100ms
moving interrupt_flag back to rb_thread_t will solve 2 and 3
1) will remain dangerous, we need to add extra checks at thread switching
because MJIT worker may get stuck if target thread stalls or dies.