Links the backtrace library which is required under FreeBSD when building llvm/lib/Support. This library is similarly linked by CMake on BSD builds at [llvm/lib/Support/CMakeLists.txt](e158b5634a/llvm/lib/Support/CMakeLists.txt (L39-L46))
Reviewed By: GMNGeoffrey
Differential Revision: https://reviews.llvm.org/D110533