x86: Wait for curthread to be set up as an indicator that the boot stack
is no longer used.

pc_curthread is set by cpu_switch after it stopped using the old
thread (or boot) stack. This makes the smp_after_idle_runnable()
function not dependent on the internals of the scheduler operations.

Reviewed by: markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D23276