[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/assert.cpp

@@ -3,7 +3,7 @@
 namespace panic {
 
 uint32_t *apic_icr = reinterpret_cast<uint32_t*>(0xffffc000fee00300);
-uintptr_t symbol_table = 0;
+void const *symbol_table = nullptr;
 
 } // namespace panic