Make libunwind build hermetic

We want to avoid exporting any symbols from Rust's version of libunwind,
and to do so we need to disable visibility annotations to make sure that
the -fvisibility=hidden has effect, and also hide global new/delete.

This matches the CMake build of libunwind.

src/libunwind/build.rs

@@ -90,6 +90,8 @@ mod llvm_libunwind {
             cfg.flag("-fstrict-aliasing");
             cfg.flag("-funwind-tables");
             cfg.flag("-fvisibility=hidden");
+            cfg.flag_if_supported("-fvisibility-global-new-delete-hidden");
+            cfg.define("_LIBUNWIND_DISABLE_VISIBILITY_ANNOTATIONS", None);
         }
 
         let mut unwind_sources = vec![