[panic] Have panics stop all cores

Kernel panics previously only stopped the calling core. This commit
re-implements the panic system to allow us to stop all cores on a panic.

Changes include:

- panic now sends an NMI to all cores. This means we can't control the
  contents of their registers, so panic information has been moved to a
  global struct, and the panicking cpu sets the pointer to that data in
  its cpu_data.
- the panic_handler is now set up with mutexes to print appropriately
  and only initialize objects once.
- copying _current_gsbase into the panic handler, and #including the
  cpprt.cpp file (so that we can define NDEBUG and not have it try to
  link the assert code back in)
- making the symbol data pointer in kargs an actual pointer again, not
  an address - and carrying that through to the panic handler
- the number of cpus is now saved globally in the kernel as g_num_cpus

src/kernel/panic.serial/display.h

@@ -5,6 +5,7 @@
 #include <stdint.h>
 
 struct cpu_state;
+struct cpu_data;
 
 namespace panicking {
 
@@ -24,6 +25,7 @@ void print_header(
         const char *file,
         uint64_t line);
 
+void print_cpu(serial_port &out, cpu_data &cpu);
 void print_callstack(serial_port &out, symbol_table &syms, frame const *fp);
 void print_cpu_state(serial_port &out, const cpu_state &regs);