Take write lock for rtld_bind before modifying obj_list in dl_iterate_phdr().

This avoids a race with readers such as dladdr(3)/dlinfo(3)/dlsym(3) and
the atexit(3) handler. This race was introduced in r294373.

Reviewed by: markj, kib, kan
MFC after: 2 weeks
Sponsored by: Dell EMC Isilon