[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/riscv/arch.c

@@ -84,7 +84,6 @@ void arch_early_init(void) {
 
 // later init per cpu
 void riscv_init_percpu(void) {
-    dprintf(INFO, "RISCV: percpu cpu num %#x hart id %#x\n", arch_curr_cpu_num(), riscv_current_hart());
 #if WITH_SMP
     // enable software interrupts, used for inter-processor-interrupts
     riscv_csr_set(RISCV_CSR_XIE, RISCV_CSR_XIE_SIE);