[panic] Don't spin the CPU in panic

While waiting for the main panicing CPU to finish, don't spin the CPU
without using `asm("pause")` to lower power consumption. Some panics can
get stuck in this state and oh man do my fans spin up.

src/kernel/panic.serial/main.cpp

@@ -30,7 +30,7 @@ void panic_handler(const cpu_state *regs)
     // If we're not running on the CPU that panicked, wait
     // for it to finish
     if (!panic) {
-        while (!main_cpu_done);
+        while (!main_cpu_done) asm ("pause");
     } else {
         new (&com1) panicking::serial_port {panicking::COM1};
         new (&syms) panicking::symbol_table {panic->symbol_data};