s390/processor: Use bitop functions for cpu flag helper functions

Use bitop functions to implement cpu flag helper functions. This way
it is guaranteed that bits cannot get lost if modified in different
contexts on a cpu.

E.g. if process context is interrupted in the middle of a
read-modify-write sequence while modifying cpu flags, and within
interrupt context cpu flags are also modified, bits can get lost.

There is currently no code which is doing this, however upcoming code
could potentially run into this problem.

Acked-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>