[kernel] Add an IPI to tell a CPU to run the scheduler

When waking another thread, if that thread has a more urgent priority
than the current thread on the same CPU, send that CPU an IPI to tell it
to run its scheduler.

Related changes in this commit:

- Addition of the ipiSchedule isr (vector 0xe4) and its handler in
  isr_handler().
- Change the APIC's send_ipi* functions to take an isr enum and not an
  int for their vector parameter
- Thread TCBs now contain a pointer to their current CPU's cpu_data
  structure
- Add the maybe_schedule() call to the scheduler, which sends the
  schedule IPI to the given thread's CPU only when that CPU is running a
  less-urgent thread.
- Move the locking of a run queue lock earlier in schedule() instead of
  taking the lock in steal_work() and again in schedule().

src/kernel/interrupts.cpp

@@ -150,6 +150,10 @@ isr_handler(cpu_state *regs)
     case isr::isrLINT1:
         break;
 
+    case isr::ipiSchedule:
+        scheduler::get().schedule();
+        break;
+
     case isr::isrSpurious:
         // No EOI for the spurious interrupt
         return;