x86: Add ffsl() and rename ffz() to ffzl() for consistency

Bit operations intended for use instead of using built-in functions.

Signed-off-by: Ivan Kolchin <ivan.kolchin@siemens.com>
[Jan: Renamed ffz placeholder for ARM as well]
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>