Add explicit @system attribute to extern functions

[pbackus]

Needed for compatibility with @safe-by-default.

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These @system-by-default function declarations were found using the deprecation message introduced in dlang/dmd#11176.

[dlang-bot]

Thanks for your pull request and interest in making D better, @pbackus! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
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Testing this PR locally

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dub run digger -- build "master + druntime#3112"