[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.h

@@ -5,6 +5,8 @@
 #include <stdint.h>
 #include <util/enum_bitfields.h>
 
+#include "interrupts.h"
+
 enum class isr : uint8_t;
 
 /// Base class for other APIC types
@@ -51,13 +53,13 @@ public:
     /// \arg mode   The sending mode
     /// \arg vector The interrupt vector
     /// \arg dest   The APIC ID of the destination
-    void send_ipi(ipi mode, uint8_t vector, uint8_t dest);
+    void send_ipi(ipi mode, isr vector, uint8_t dest);
 
     /// Send an inter-processor broadcast interrupt to all other CPUs
     /// \arg mode   The sending mode
     /// \arg self   If true, include this CPU in the broadcast
     /// \arg vector The interrupt vector
-    void send_ipi_broadcast(ipi mode, bool self, uint8_t vector);
+    void send_ipi_broadcast(ipi mode, bool self, isr vector);
 
     /// Wait for an IPI to finish sending. This is done automatically
     /// before sending another IPI with send_ipi().