lttng-libust: Enable aarch64 build

With aarch64 support added to liburcu, lttng-libust has its dependencies met
for aarch64, so allow it to build for this architecture.

Signed-off-by: Ben Shelton <benjamin.h.shelton@intel.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>