[GDC] GCC9 last-minute fix: MinGW changes to get phobos to build

[jpf91]

This should hopefully contain only uncontroversial changes required to build phobos for MinGW. We have to hurry a bit to get this still into GCC-9, as that will likely be released next friday.

Related druntime PR: dlang/druntime#2586
Related GDC PR: D-Programming-GDC/gcc#14

ping @ibuclaw @redstar

[dlang-bot]

Thanks for your pull request and interest in making D better, @jpf91! 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 fetch digger
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[jpf91]

Thanks a lot everyone!