[kernel] Add a _very_ basic TLB shootdown mechanism

This TLB shootdown implementation is pretty janky: When the kernel
unmaps a mapping, it sends an IPI to all other cores and doesn't even
wait to ensure they've finished handling it. Upon getting one of these
IPIs, the core just re-writes cr3 to flush all TLBs.

src/kernel/interrupts.cpp

@@ -21,6 +21,7 @@ constexpr uintptr_t apic_eoi_addr = 0xfee000b0 + mem::linear_offset;
 extern "C" {
     void isr_handler(cpu_state*);
     void irq_handler(cpu_state*);
+    void _reload_cr3();
 }
 
 uint8_t
@@ -187,6 +188,11 @@ isr_handler(cpu_state *regs)
         scheduler::get().schedule();
         break;
 
+    case isr::ipiShootdown:
+        // TODO: Real shootdown algorithm
+        _reload_cr3();
+        break;
+
     default:
         util::format({message, sizeof(message)}, "Unknown interrupt 0x%lx", regs->interrupt);
         kassert(false, message, regs);