liburcu: only some architectures are supported

Make sure that liburcu (and the packages that select it) cannot be
enabled on the architectures that are not supported. At the moment,
only x86, x86-64, PowerPC and ARM are supported.

[Peter: add armeb as well]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>