BinUtils: Restore llvm-ar fallback on Apple platforms

Brad Kingrequested to merge
brad.king/cmake:binutils-llvm-ar-apple into master

Since !7039 (merged) we do not consider llvm-ar at all on Apple platforms. However, there are existing cross-compiling use cases in which the toolchain has llvm-ar but not ar. Prior to the re-ordering in !6164 (merged), we preferred ar and then llvm-ar. Restore the original order for Apple.

Fixes: #23320 (closed)
Issue: #23333 (closed)
Backport: release

Edited by Brad King

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