[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/cpu.cpp

@@ -1,4 +1,5 @@
 #include <stdint.h>
+#include <string.h>
 
 #include "assert.h"
 #include "cpu.h"
@@ -12,6 +13,11 @@
 #include "syscall.h"
 #include "tss.h"
 
+unsigned g_num_cpus = 1;
+
+panic_data g_panic_data;
+panic_data *g_panic_data_p = &g_panic_data;
+
 cpu_data g_bsp_cpu_data;
 
 void
@@ -37,9 +43,18 @@ cpu_validate()
 #undef CPU_FEATURE_REQ
 }
 
+void
+global_cpu_init()
+{
+    memset(&g_panic_data, 0, sizeof(g_panic_data));
+}
+
 void
 cpu_early_init(cpu_data *cpu)
 {
+    if (cpu->index == 0)
+        global_cpu_init();
+
     cpu->idt->install();
     cpu->gdt->install();