s390/bitops: Switch to generic ffs() if supported by compiler

Use generic ffs() / __builtin_ffs() if supported by the compiler. GCC 16
will have support for __builtin_ffs().
See gcc commit f50cff9766c5 ("s390: Implement clz and ctz for SI mode").

In the distant future when GCC 16 becomes the minimum supported version,
this allows to get rid of the flogr inline assembly.

Kernel image size is reduced by ~500 bytes (gcc 16 beta + 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>