arm64: Disable outling atomics

We don't have the symbols for this. The virtio randon number driver
uses a C11 atomic operation. With inline atomics this is translated to
an Armv8.0 atomic operation, with outling atomics this becomes a
function call to a handler. As we don't have the needed function the
kernel fails to link.

Fix by disabling outline atomics for now.

Reviewed by: brooks, imp
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D45301