Blender Git Loki

Add locking fallback atomics implementation

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