[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/main.cpp

@@ -82,7 +82,7 @@ kernel_main(bootproto::args *args)
 {
     if (args->panic) {
         IDT::set_nmi_handler(args->panic->entrypoint);
-        panic::symbol_table = args->symbol_table | mem::linear_offset;
+        panic::symbol_table = util::offset_pointer(args->symbol_table, mem::linear_offset);
     }
 
     init_console();
@@ -149,7 +149,7 @@ kernel_main(bootproto::args *args)
     apic->calibrate_timer();
 
     const auto &apic_ids = devices.get_apic_ids();
-    unsigned num_cpus = start_aps(*apic, apic_ids, args->pml4);
+    g_num_cpus = start_aps(*apic, apic_ids, args->pml4);
 
     interrupts_enable();
     g_com1.handle_interrupt();
@@ -178,7 +178,7 @@ kernel_main(bootproto::args *args)
     }
     */
 
-    scheduler *sched = new scheduler {num_cpus};
+    scheduler *sched = new scheduler {g_num_cpus};
     scheduler_ready = true;
 
     // Load the init server