gn build: Use -fvisibility-global-new-delete=force-hidden to build libcxx/libcxxabi/libunwind.

-fvisibility-global-new-delete-hidden is deprecated and clang was warning
about it on every build command. These libraries are always built using
a stage2 compiler, so we can use the new build flag unconditionally.

Reviewers: aeubanks

Reviewed By: aeubanks

Pull Request: https://github.com/llvm/llvm-project/pull/88459