BinUtils: Avoid llvm-ar on Apple platforms

Since commit cf82300a63 (BinUtils: Clarify search logic and make it more
consistent, 2021-05-27, v3.21.0-rc1~119^2~2) we correctly prefer the
more-specific name `llvm-ar` over `ar` when using Clang.  However, on
Apple platforms, `llvm-ar` does not generate a symbol table that the
Apple linker accepts.  Fall back to `ar` on Apple platforms.

Fixes: #23269