kernel/smp: Fix races in SMP initialization

This has bitrotten a bit.  Early implementations had a synchronous
arch_start_cpu(), but then we started allowing that to be an async
operation.  But that means that CPU start now becomes surprisingly
reentrant to the arch layer (cpu 0 can get a call to start cpu 2 while
cpu 1's initialization code is still running).  That's just error
prone; we never documented the requirements cleanly (the window is
very small, but not so small to a slow simulator!).

Add an extra flag so we don't issue the next start until the last is
out of the arch layer and running in smp_init_top().

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>