[Bazel] add linkopt to warn on unresolved symbols for llvm/lib/Support under FreeBSD

[wamuir]

Summary

PR adds -Wl,--warn-unresolved-symbols to linker flags for Bazel builds of llvm/lib/Support under FreeBSD. This avoids an error on an unresolved reference to environ. As far as I can tell, on FreeBSD, environ is defined as a common symbol in /usr/lib/crt1.o, and won't be resolved during linking. Ref extern char **environ in llvm/lib/Support/Unix/Program.inc.

[GMNGeoffrey]

Hmmm I rather suspect this isn't the right fix. Can you provide the error message you're hitting? I don't see any place that the CMake build has --warn-unresolved-symbols, so I suspect we're setting something wrong in the config.

[wamuir]

Completely agree. This is a workaround and the issue appears to be specific to builds in Bazel/FreeBSD. For context, I'm encountering while building libtensorflow on FreeBSD 13.0.

I'll update shortly with the exact error message and additional details.

[wamuir]

Closing. This is a build environment issue; error is ld: error: undefined symbol: environ and is the result of a default invocation of linker flag -z defs one of the tensorflow bazel build rules

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

     includes = ["include"],
     linkopts = select({
         "@bazel_tools//src/conditions:windows": [],
+        "@bazel_tools//src/conditions:freebsd": [
+            "-pthread",
+            "-ldl",
+            "-lm",
+            "-Wl,--warn-unresolved-symbols",
+        ],
         "//conditions:default": [
             "-pthread",
             "-ldl",