MinGW 6.0.0 libraries for DMD

[Seb]

Hi all,

so I just revitalized the half-dead PR from kinke [1] which 
upgrades DMD's MinGW libraries to MinGW-64 6.0.0.

The bundle is now available at [2] and it would be awesome if the 
people actually interested in this could give it a quick test:

1) Download the built MinGW libaries from [2]
2) Replace them with dmd2/windows/lib64/mingw and/or 
dmd2/windows/lib32mscoff in your DMD Windows installation
3) Test whether everything is still working

Ideally we could get this in before the upcoming 2.086.0 beta 
(about to branch off in two days), s.t. the Windows folk who were 
running into problems with the older MinGW 5 libraries can use 
this officially starting from May.

The PR to switch the installer/release to use MinGW 6.0.0 is 
available at [3].

Disclaimer: I don't use Windows, but I do hear that people 
complain about it too often.

Cheers,

Seb

[1] https://github.com/dlang/installer/pull/346
[2] http://downloads.dlang.org/other/mingw-libs-6.0.0.zip
[3] https://github.com/dlang/installer/pull/377