[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/smp.h

@@ -0,0 +1,18 @@
+#pragma once
+/// \file smp.h
+/// Multi-core / multi-processor startup code
+
+struct cpu_data;
+
+namespace smp {
+
+/// Start all APs and have them wait for the BSP to
+/// call smp::ready().
+/// \arg bsp    The cpu_data struct representing the BSP
+/// \arg kpml4  The kernel PML4
+unsigned start(cpu_data &bsp, void *kpml4);
+
+/// Unblock all APs and let them start their schedulers.
+void ready();
+
+} // namespace smp