[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](e158b5634a/llvm/lib/Support/CMakeLists.txt (L39-L46))

Reviewed By: GMNGeoffrey

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

utils/bazel/llvm-project-overlay/llvm/BUILD.bazel

@@ -206,6 +206,12 @@ cc_library(
     includes = ["include"],
     linkopts = select({
         "@bazel_tools//src/conditions:windows": [],
+        "@bazel_tools//src/conditions:freebsd": [
+            "-pthread",
+            "-lexecinfo",
+            "-ldl",
+            "-lm",
+        ],
         "//conditions:default": [
             "-pthread",
             "-ldl",