fix Clang LTO archiver selection on Apple platforms

[mercury233]

What does this PR do?

Fixes Clang LTO builds on Apple platforms where Premake incorrectly selected llvm-ar, which is not provided by the default Apple toolchain. The Apple toolchain already uses LLVM, but the executable name is ar.

How does this PR change Premake's behavior?

On macOS, Premake now uses the system ar for static libraries. Other versioned toolsets, such as clang-16, retain the existing behavior. Other platforms continue to use llvm-ar when LTO is enabled.

Anything else we should know?

I just made my project build pass with LTO on, didn't test it fully.

Did you check all the boxes?

• Focus on a single fix or feature; remove any unrelated formatting or code changes
• Add unit tests showing fix or feature works; all tests pass
• Mention any related issues (put closes #XXXX in comment to auto-close issue when PR is merged)
• Follow our coding conventions
• Minimize the number of commits
• Align documentation to your changes

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