ARROW-12485: [C++] Use mimalloc as the default memory allocator on macOS
ARROW-12485: [C++] Use mimalloc as the default memory allocator on macOS

Closes #10117 from jonkeane/ARROW-12485

Authored-by: Jonathan Keane <28d17c45cab2bb8b25bea44a915713e26ce37e1b@gmail.com>
Signed-off-by: Krisztián Szűcs <6bab32dc957a05a87930e779f272342e159326fa@gmail.com>

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