package/lttng-libust: use BR2_PACKAGE_LIBURCU_ARCH_SUPPORTS dependency

Using this newly introduced symbol lets to simplify the dependency code.

This change also adds the missing architecture dependencies on the second
comment.

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>