Add easy to use spin lock primitive
Add easy to use spin lock primitive

Currently unused, but will be handy for an upcoming changes.

It'll also be nice to be able to do scoped_lock() for both
Mutex and Spin, but currently it's not really easy to do,
need some changes in typedefs and such, will happen as a
separate commit.

tangent-opensource/coreBlackbird,tangent-opensource/coreBlackbird,tangent-opensource/coreBlackbird