x86: unify power/cpu_(32|64) global variables

Aiming total unification of cpu_32.c and cpu_64.c, in this step
we do unify the global variables and existing forward declarations
for such files.

Signed-off-by: Sergio Luis <sergio@larces.uece.br>
Signed-off-by: Lauro Salmito <laurosalmito@gmail.com>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>