[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.h

@@ -18,6 +18,20 @@ struct cpu_state
     uint64_t rip, cs, rflags, rsp, ss;
 };
 
+/// Kernel-wide panic information
+struct panic_data
+{
+    void const * symbol_data;
+    char const * message;
+    char const * function;
+    char const * file;
+    uint32_t line;
+    uint16_t cpus;
+};
+
+extern unsigned g_num_cpus;
+extern panic_data *g_panic_data_p;
+
 /// Per-cpu state data. If you change this, remember to update the assembly
 /// version in 'tasking.inc'
 struct cpu_data
@@ -39,6 +53,7 @@ struct cpu_data
     // Members beyond this point do not appear in
     // the assembly version
     lapic *apic;
+    panic_data *panic;
 };
 
 extern "C" cpu_data * _current_gsbase();