[mp] restructure the sequence of how cpus are brought up

- Move a bit of the shared logic of secondary bootstrapping into a new
  function, lk_secondary_cpu_entry_early() which sets the current cpu
  pointer before calling the first half of the secondary LK_INIT
  routines.
- Create the per cpu idle threads on the main cpu instead of the
  secondary as they come up.
- Tweak all of the SMP capable architectures to use this new path.
- Move the top level mp routines into a separate file top/mp.c
- A bit more correctly ifdef out more SMP code.

arch/x86/mp.c

@@ -96,11 +96,12 @@ void x86_secondary_entry(uint cpu_num) {
 
     x86_early_init_percpu();
 
-    // run early secondary cpu init routines up to the threading level
-    lk_init_level(LK_INIT_FLAG_SECONDARY_CPUS, LK_INIT_LEVEL_EARLIEST, LK_INIT_LEVEL_THREADING - 1);
+    // Get us into thread context and run the initial secondary cpu init routines
+    lk_secondary_cpu_entry_early();
 
-    dprintf(INFO, "SMP: secondary cpu %u started, apic id %u\n", arch_curr_cpu_num(), apic_id);
+    dprintf(INFO, "X86: secondary cpu %u started, apic id %u\n", arch_curr_cpu_num(), apic_id);
 
+    // Finish secondary cpu initialization and enter the scheduler
     lk_secondary_cpu_entry();
 
     // should never get here except for an error condition