Impact: Minor optimization.

Implement change_bit with immediate bit count as "lock xorb". This is
similar to  "lock orb" and "lock andb"  for set_bit and clear_bit
functions.
Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>