zdtm: fix atomic_{sub, add}_return assembler for aarch64

Running the test suite on aarch64 showed two tests failing (static/fd
and transition/maps0007). Debugging showed that the
atomic_{sub,add}_return() functions did not return the value but the
lower 32bit of the address of that value.

Simply dereferencing the input solves the two mentioned test cases.

Signed-off-by: Dmitry Safonov <dima@arista.com>
Signed-off-by: Adrian Reber <areber@redhat.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>