[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.h

@@ -44,6 +44,11 @@ inline void panic(
     while (1) asm ("hlt");
 }
 
+/// Install a panic handler.
+/// \arg entrypoint   Virtual address of the panic handler's entrypoint
+/// \arg symbol_data  Pointer to the symbol table data
+void install(uintptr_t entrypoint, const void *symbol_data);
+
 } // namespace panic
 
 #ifndef NDEBUG