[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/idt.h

@@ -36,7 +36,7 @@ public:
     void set_ist(uint8_t i, uint8_t ist) { m_entries[i].ist = ist; }
 
     /// Get the IST entries that are used by this table, as a bitmap
-    uint8_t used_ist_entries() const;
+    static uint8_t used_ist_entries();
 
     /// Dump debug information about the IDT to the console.
     /// \arg index  Which entry to print, or -1 for all entries