[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.

dev/timer/arm_generic/arm_generic_timer.c

@@ -327,7 +327,7 @@ static void arm_generic_timer_init_secondary_cpu(uint level) {
 /* secondary cpu initialize the timer just before the kernel starts with interrupts enabled */
 LK_INIT_HOOK_FLAGS(arm_generic_timer_init_secondary_cpu,
                    arm_generic_timer_init_secondary_cpu,
-                   LK_INIT_LEVEL_THREADING - 1, LK_INIT_FLAG_SECONDARY_CPUS);
+                   LK_INIT_LEVEL_PLATFORM_EARLY, LK_INIT_FLAG_SECONDARY_CPUS);
 
 static void arm_generic_timer_resume_cpu(uint level) {
     /* Always trigger a timer interrupt on each cpu for now */