[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/kernel.module

@@ -10,7 +10,6 @@ kernel = module("kernel",
     includes = [ "." ],
     sources = [
         "apic.cpp",
-        "ap_startup.s",
         "assert.cpp",
         "boot.s",
         "clock.cpp",
@@ -45,6 +44,8 @@ kernel = module("kernel",
         "pci.cpp",
         "printf/printf.c",
         "scheduler.cpp",
+        "smp.cpp",
+        "smp.s",
         "syscall.cpp.cog",
         "syscall.h.cog",
         "syscall.s",