ntdll: Treat XDG_DATA_DIRS as special env variable.
Wine-Bug: https://bugs.winehq.org/show_bug.cgi?id=49987
GTK apps seem to use this env var to find path to schemas, like Meld: https://download.gnome.org/binaries/win32/meld/3.22/Meld-3.22.2-mingw.msi which doesn't start at all without this patch. More info in https://bugs.winehq.org/show_bug.cgi?id=49987.
v2: add WINEXDG_DATA_DIRS and use that in winemenubuilder.