[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/panic.serial/main.cpp

@@ -58,8 +58,7 @@ void panic_handler(const cpu_state *regs)
 
     // If we're running on the CPU that panicked, tell the
     // others we have finished
-    if (panic)
-        main_cpu_done = true;
+    main_cpu_done = true;
 
     if (__atomic_sub_fetch(&remaining, 1, order) == 0) {
         // No remaining CPUs, if we're running on QEMU,