s390: provide word-at-a-time implementation

s390: provide word-at-a-time implementation

Provide an s390 specific word-at-a-time implementation. Compared to the
generic variant the generated code for has_zero() is slightly
better. However find_zero() is much simpler since it reuses the result
of __fls() aka flogr() and now comes without any conditional branches,
while the generic variant has three of them.

Reviewed-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>