rtld: implement _dl_iterate_phdr_locked

Some sanitizers need to be able to use dl_iterate_phdr() after stopping
the rest of the process, but it's very hard to do so reliably as a
non-participant in the main logic of the program.

Introduce _dl_iterate_phdr_locked to bypass the locking that's normally
required for dl_iterate_phdr() and slap some scary warning on it. It
will remain undocumented and probably shouldn't be used for anything
else.

Reviewed by: kib

(cherry picked from commit 1426fd6cff0603f0ee275b99f2ba35dc36f3d0c2)