Don't use "unadjusted" ABI on non windows platforms
Don't use "unadjusted" ABI on non windows platforms

We introduced the unadjusted ABI to work around wrong
(buggy) ABI expectations by LLVM on Windows [1].
Therefore, it should be solely used on Windows and not
on other platforms, like right now is the case.

[1]: see this comment for details https://github.com/rust-lang/rust/pull/38482#issuecomment-269074031

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