s390/zdtm: Also support z900 (debian)

Debian is compiled for z900 - therefore we can't use the "laa"
instruction.

Fix this and replace "laa" by the older "cs" (compare and swap) loop.

Signed-off-by: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>