[PATCH v2] __libdw_dieabbrev: Replace rwlock with __atomic builtins

[Frank Ch. Eigler]

Hi -

> > Maybe we just have to say that you cannot share Dwarf_Die references
> > between threads? So if you want to use a Dwarf_Die from another thread
> > you have to copy it instead by passing it by reference? [...]

(Exactly.  The Dwarf_Die object is small enough, 4 pointers, to be
simply copied in toto.)

- FChE