s390: Provide optimized __arch_hweight*() implementations

Make use of the popcnt instruction to provide optimized __arch_hweight*()
implementations. The generated code is shorter and avoids rather expensive
functions calls.

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>