libatomic_ops: make it available only on supported architectures

libatomic_ops build fails on architectures such as MIPS or SuperH that
are not supported. So we make it possible to select the libatomic_ops
package only for the architectures that are known to be supported.

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