x86/cpu: Fix typos and improve the comments in sync_core()

- Fix typos.

- Move the compiler barrier comment to the top, because it's valid for the
  whole function, not just the legacy branch.

Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20200818053130.GA3161093@gmail.com


Reviewed-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>