apache-arrow: restore correct minimum CPU

[cho-m]

ARROW_SIMD_LEVEL is the minimum CPU and on x86_64 our superenv cannot discard -msse4.2 so using default can lead to illegal instructions on Tier 1 setups.

Can ignore special Intel macOS handling given short lifespan and Apache Arrow will still runtime dispatch to SSE4.2.

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Also remove ARROW_MIMALLOC which is default so conditional doesn't do anything:

• https://github.com/apache/arrow/blob/apache-arrow-25.0.1/cpp/cmake_modules/DefineOptions.cmake#L364

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