`expand_cxxstring_abis` for FreeBSD and MacOS

[vchuravy]

@staticfloat I recall having a conversation which you recently about building LLVM, where
we were confused about the fact that both Mac and FreeBSD got their cxxstring ABIs expanded.

As we can see here https://github.com/JuliaBinaryWrappers/LLVM_jll.jl/releases that is indeed the case and it causes issues with JuliaLang/julia#34418 since the base infrastructure doesn't seem to expect the cxxstring ABI on FreeBSD or Mac.

[staticfloat]

We just need to tell the Make system to always download the cxx03 or cxx11 build. Because we build with clang on those platforms, the extra defines we add don't change anything (it would if we used g++ or gcc in the build, but we know we don't).

[vchuravy]

Fixed upstream.