package/liburcu: introduce BR2_PACKAGE_LIBURCU_ARCH_SUPPORTS symbol

This new hidden symbol will allow simplifying the architecture dependencies
on package that depends on liburcu. These packages will be updated in follow-up
patches.

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>