Add -de to posix.mak

[JinShil]

Revival of #1982

Per @wilzbach

This is already the way we build Phobos + Tools.
See also: dlang/phobos#5546

[dlang-bot]

Thanks for your pull request and interest in making D better, @JinShil! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
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• My PR is fully covered with tests (you can see the annotated coverage diff directly on GitHub with CodeCov's browser extension
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• I have provided a detailed rationale explaining my changes
• New or modified functions have Ddoc comments (with Params: and Returns:)

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Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub fetch digger
dub run digger -- build "master + druntime#2542"

[JinShil]

Closing in favor of new commits to #1982 for now.