zfs: lz4: Recognize __aarch64__ when detecting CPU word size.

The lz4 code checks from a predetermined list of definitions
if the CPU word size is 64. Otherwise, it's 32.
Therefore, __aarch64__ definition must be added into the afore-
mentioned list.

Signed-off-by: Raphael S. Carvalho <raphaelsc@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>