libthr: Reorder pthread mutex to prevent false sharing

[aokblast]

Summary

On a 64 byte cache line CPU (amd64), m_owner is contended by all of
the shared processes. However, after finding one winner, mtx is
enqueued, which trigger a cache refresh from other processors. Reorder
mtx so that m_lock and m_qu will not be in the same cache line.