[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/objects/thread.cpp

@@ -53,7 +53,17 @@ thread::block()
 void
 thread::wake(uint64_t value)
 {
+    if (has_state(state::ready))
+        return;
+
     m_wake_value = value;
+    wake_only();
+    scheduler::get().maybe_schedule(tcb());
+}
+
+void
+thread::wake_only()
+{
     m_wake_timeout = 0;
     set_state(state::ready);
 }