s390/bitops: find leftmost bit instruction support

The flogr instruction scans a bitmap starting from the leftmost bit.
Implement support for these bitops. This could be useful to scan
bitmaps like an interrupt vector set by the hardware starting
at the leftmost bit.

Signed-off-by: Jan Glauber <jang@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>