[Bazel] link backtrace library when building llvm/lib/Support under FreeBSD

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](https://github.com/llvm/llvm-project/blob/e158b5634aa67ea3039a62c3d8bda79b77b3b21c/llvm/lib/Support/CMakeLists.txt#L39-L46)

Reviewed By: GMNGeoffrey

Differential Revision: https://reviews.llvm.org/D110533