Always include all the sources as we cannot include object libraries to be
subsumed. This allows us to conditionally build the swift runtime for foreign
hosts simultaneously (e.g. cross compile for Linux and Windows simultaneously).
- added read / write lock support
- added non-fatal error support to allow use of mutex in fatal error reporting pathway
- isolated pthread implementation to it own header/cpp file pair
- expanded unit tests to cover new code as well as better test existing mutex
- removed a layer of complexity that added no real value
