x86: Remove special-case paths for start-of-day SMP bringup.

x86: Remove special-case paths for start-of-day SMP bringup.

All CPUs are now brought up in __cpu_up(), in the correct order for
cpu-bringup notifiers.

Notably ganged TSC calibration is removed. This was another
start-of-day-specific aspect of bringup. If we really need
calibration, I think we can come up with an efficient method for APs
to calibrate themselves against PIT during their bringup (even
asynchronously, if required for efficiency).

Signed-off-by: Keir Fraser <keir.fraser@citrix.com>