bitops: use __fls for fls64 on 64-bit archs

Use __fls for fls64 on 64-bit archs. The implementation for
64-bit archs is moved from x86_64 to asm-generic.

Signed-off-by: Alexander van Heukelum <heukelum@fastmail.fm>
Signed-off-by: Ingo Molnar <mingo@elte.hu>