In the light of roland's comments, I've checked in the attached 
variation on the original patch.

It still does the conversion but in GDB's libthread_db caller 
(enable_thread_event_reporting) and not in libthread_db's symbol lookup 
callee (ps_pglobal_lookup).

This way, libthread_db is free to search for either:
.__nptl_create_event: the start address
__nptl_create_event: the descriptor
(the original change would have restricted searches to just the start 
address - not a problem now but we never know) and at the same time 
ensure that GDB sets breakpoints at the address it needs.

Andrew