package/libatomic_ops: rename libatomic_ops supported arch Kconfig symbol

According to the discussion on the mailing-list [1], rename
the libatomic_ops supported architectures Kconfig symbol.

[1] http://lists.busybox.net/pipermail/buildroot/2016-February/152146.html



Signed-off-by: Romain Naour <romain.naour@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Frank Hunleth <fhunleth@troodon-software.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>