Is used for platforms for which we do not have native implementation,
such as MIPS, for example.
It is possible to test locking implementation on local computer by
defining `ATOMIC_FORCE_USE_FALLBACK` in the atomic_ops_unix.h file.
Run full regression suit with the locking implementation on amd64
Linux platform and all tests passed.
Having non-optimal but working implementation for odd-ball platforms
seems to be a better choice than failing the build entirely.
Differential Revision: https://developer.blender.org/D12313
1bb2077250