s390/bitops: Switch to generic fls(), fls64(), etc.

Switch to generic fls(), fls64(), etc. which are implemented with
__builtin_ctzl(), __builtin_clzl().
Those builtins are available for all supported compilers.

Kernel image size is reduced by ~10kb (gcc 15.1.0 + defconfig).

Acked-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>