[Sanitizers] Get link map on FreeBSD via documented API

Summary:
Instead of hand-crafting an offset into the structure returned by
dlopen(3) to get at the link map, use the documented API.  This is
described in dlinfo(3): by calling it with `RTLD_DI_LINKMAP`, the
dynamic linker ensures the right address is returned.

Reviewers: devnexen, emaste, MaskRay, krytarowski

Reviewed By: krytarowski

Subscribers: krytarowski, vitalybuka, #sanitizers, llvm-commits

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D73990