x86: unify power/cpu_(32|64) regarding restoring processor state

In this step we do unify cpu_32.c and cpu_64.c functions that
work on restoring the saved processor state. Also, we do
eliminate the forward declaration of fix_processor_context()
for X86_64, as it's not needed anymore.

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>