s390/processor: add test_and_set_cpu_flag() and test_and_clear_cpu_flag()

Add test_and_set_cpu_flag() and test_and_clear_cpu_flag() helper functions.

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