Adding Msan support to FreeBSD

Summary:
Enabling the memory sanitizer support for FreeBSD, most of unit tests are compatible.
- Adding fstat and stressor_r interceptors.
- Updating the struct link_map access since most likely the struct Obj_Entry had been updated since.
- Disabling few unit tests until further work is needed (or we can assume it can work in real world code).

Patch by: David CARLIER

Reviewers: vitalybuka, krytarowski

Reviewed By: vitalybuka

Subscribers: eugenis, dim, srhines, emaste, kubamracek, mgorny, fedor.sergeev, hintonda, llvm-commits, #sanitizers

Differential Revision: https://reviews.llvm.org/D43080

llvm-svn: 326644