macOS: CMake may generate invalid static libraries using Clang

When llvm-ar is available in PATH, CMake will prefer that over ar (assuming a LLVM based toolchain IIRC). On Darwin platforms, there is no llvm-ar available by default so this is unlikely to occur. However, should it occur, the use of a different librarian causes issues as llvm-ar does not generate a symbol table which ld64 deems cromulent. Attempting to link with the resulting library will fail with an obscure error message:

ld: warning: ignoring file ..., building for macOS-x86_64 but attempting to link with file built for macOS-x86_64

Ideally, we should be using libtool to generate the static library on Darwin as that is the officially supported librarian in cctools.