Migrate from MinGW to mingw-w64

Important: depends on !1 (merged) to be merged first but can be tested independently

Windows x86-64 builds are successful but yet untested (same as linux x86-64)

I copied test builds to: einstein:/BOINC/projects/EinsteinAtHome/apps/gfx (win64 and lin64)

FYI, until !1 (merged) gets merged this MR will also show those changes/commits which doesn't help reviewing them in isolation. Thus I recommend ignoring this MR until after !1 (merged) got merged.

merged