[Sanitizers] Porting getrandom/getentropy interceptors to FreeBSD

- Available from 12.x branch, by the time it lands next year in FreeBSD tree, the 11.x's might be EOL.
- Intentionally changed the getrandom test to C code as with 12.0 (might be fixed in CURRENT since), there is a linkage issue in C++ context.

Reviewers: emaste, dim, vitalybuka

Reviewed-By: vitalybuka

Differential Revision: https://reviews.llvm.org/D68451

llvm-svn: 374315