BinUtils: Restore llvm-ar fallback on Apple platforms

Since commit 29ea94e17c (BinUtils: Avoid llvm-ar on Apple platforms,
2022-03-03, v3.21.6~1^2) 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 commit cf82300a63 (BinUtils: Clarify search logic and
make it more consistent, 2021-05-27, v3.21.0-rc1~119^2~2), we preferred
`ar` and then `llvm-ar`.  Restore the original order for Apple.

Fixes: #23320