Adds an rstat "synchs_not_at_safe_spot" measuring the count of synch_with_thread attempts that had to be re-tried due to the target being suspended at an un-translatable (i.e., unsafe) spot.
Adds a corresponding dr_stats_t field "synchs_not_at_safe_spot" for extraction.
These will be useful for measuring the impact of clean calls and other un-translatable places on detach and other synch operations.
Issue: #4219