[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/apic.cpp

@@ -71,7 +71,7 @@ lapic::get_id()
 }
 
 void
-lapic::send_ipi(ipi mode, uint8_t vector, uint8_t dest)
+lapic::send_ipi(ipi mode, isr vector, uint8_t dest)
 {
     // Wait until the APIC is ready to send
     ipi_wait();
@@ -85,7 +85,7 @@ lapic::send_ipi(ipi mode, uint8_t vector, uint8_t dest)
 }
 
 void
-lapic::send_ipi_broadcast(ipi mode, bool self, uint8_t vector)
+lapic::send_ipi_broadcast(ipi mode, bool self, isr vector)
 {
     // Wait until the APIC is ready to send
     ipi_wait();