Fix crash due to atomics on bare-metal arm64

Fix crash due to atomics on bare-metal arm64

At the moment atomic operations are working on ARM64 for KVM; however,
they are not working on baremetal on ARM64 (i.e., on the Raspberry Pi
3B+).  This can be most likely be solved by enabling the caches and
setting up a compatible memory configuration.  In the meantime, these
patches allow us to have functionally-equivalent non-atomic
implementations of the gcc builtins working on baremetal on ARM64.
Once the issue is fixed this workaround can be removed.

Signed-off-by: Felipe Huici <felipe.huici@neclab.eu>
Reviewed-by: Simon Kuenzer <simon.kuenzer@neclab.eu>