Enable GNU ar tool selection on MacOS

[theComputeKid]

• select gcc-ar rather than Apple ar (which is neither llvm or gnu).
• there is no point using "gcc" as a toolset on macos as it symlinks to clang, and changing that symlink to actual GNU is dangerous. Premake treats "gcc" as a GNU toolchain and passes GNU flags, which may not apply to apple clang.
• probably macos users have added gcc-N to path and that is what they mean when they explicitly specify --cc=gcc-N (e.g. gcc-16). This is what homebrew does. In which case, this change is reflective of the actual intentions of the user to use the real GNU toolchain.

I am not sure what to do. Making this change means that it might break compilation for users who rely on this behaviour:

$ gcc --version  
Apple clang version 21.0.0 (clang-2100.1.1.101)
Target: arm64-apple-darwin25.5.0
Thread model: posix
InstalledDir: /Library/Developer/CommandLineTools/usr/bin

Fixes #2773