[stable] Fix -mixin output for CRLF input

[kinke]

The mixin string may contain Windows newlines, e.g., if the source file does and a multiline q{ ... } literal is used (e.g., test/runnable/extra-files/mixin.d). In that case, the old code would output CRLF on POSIX and CRCRLF on Windows instead of normalizing the output (LF/CRLF).

[dlang-bot]

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Testing this PR locally

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