[kernel] Clean up main.cpp and others

The kernel/main.cpp and kernel/memory_bootstrap.cpp files had become
something of a junk drawer. This change cleans them up in the following
ways:

- Most CPU initialization has moved to cpu.cpp, allowing several
  functions to be made static and removed from cpu.h
- Multi-core startup code has moved to the new smp.h and smp.cpp, and
  ap_startup.s has been renamed smp.s to match.
- run_constructors() has moved to memory_bootstrap.cpp, and all the
  functionality of that file has been hidden behind a new public
  interface mem::initialize().
- load_init_server() has moved from memory_bootstrap.cpp to main.cpp

src/kernel/assert.cpp

@@ -1,10 +1,18 @@
 #include "assert.h"
+#include "idt.h"
 
 namespace panic {
 
 uint32_t *apic_icr = reinterpret_cast<uint32_t*>(0xffffc000fee00300);
 void const *symbol_table = nullptr;
 
+void
+install(uintptr_t entrypoint, const void *symbol_data)
+{
+    IDT::set_nmi_handler(entrypoint);
+    symbol_table = symbol_data;
+}
+
 } // namespace panic
 
 extern "C"