libatomic_ops: introduce a BR2_PACKAGE_LIBATOMIC_ARCH_SUPPORTS hidden option

In preparation to the introduction of packages that select
libatomic_ops, this commit adds an hidden
BR2_PACKAGE_LIBATOMIC_ARCH_SUPPORTS option which allows to easily
depend on the architectures that libatomic_ops is available for.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>