To my knowledge there isn't an implementation of `sse4.2` that doesn't have `crc32`.
The Clang driver also sets `crc32` to be implicitly enabled when an explicit `-crc32`
wasn't provided. This matches that behaviour.
We need this behaviour to compile libraries like `rocksdb` which currently guard against
`crc32` intrinsics by checking for `sse4.2`.
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