Reuse `sys::unix::cmath` on other platforms
Reuse `sys::unix::cmath` on all non-`windows` platforms.
`unix` is chosen as the canonical location instead of `unsupported` or `common` because `unsupported` doesn't make sense semantically and `common` is reserved for code that is supported on all platforms. Also `unix` is already the home of some non-`windows` code that is technically not exclusive to `unix` like `unix::path`.
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