Fully backport XP target code

[K4sum1]

This would fix x64 builds compiling with the 6.00 target because 5.01 is "invalid" and the Microshit linker defaults to 6.00 instead of just using the specified target.

qt/qtbase@0e1efdf

[K4sum1]

Might also have to revert this for mingw. I compile with msvc though so it's not a priority.

qt/qtbase@d57a7c4