make Thread.sm_this a plain TLS value

redo #456, the underlying bug was fixed by #458

Revert "Merge pull request #457 from AndrejMitrovic/Revert456"

This reverts commit c50f054611e30630c1c6e6be951da3df631909a2, reversing
changes made to dee2d7576ca94a21706f64d83ff9b0dfe761a7b1.