Upgrade MinGW-based libs to MinGW-w64 v8.0.0

[kinke]

I've compared the crucial {msvcr*,ucrtbase}.def.in files against v7.0.0, and they don't seem to feature any relevant changes.

See http://mingw-w64.org/doku.php/versions for the changelog; Direct3D 12 is one of the additions.

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[PetarKirov]

LGTM
@rainers can you please take a look?

[rainers]

Had a cursory look at the artifact, looks good. One library name was corrected when compared to the Windows SDK (rstrmgr.lib -> rstrtmgr.lib).
To make it into the installer, we need to make a release here and update build_all.d.

[wilzbach]

Github release for resource caching: https://github.com/dlang/installer/releases/tag/mingw-libs-8.0.0
GitHub build_all.d PR: #475