The "contract" for toolchains is that extracting foo.tar.xz creates a
directory named foo/. That is however not true for mingw32.tar.xz, which
extracts into gcc/, possibly overwriting files from the gcc.tar.xz
archive (which is also used for mingw builds, for the host part).
This is also not true for nsis.tar.xz, but it reportedly has problems
when it's not in the same directory as mingw32.
But mingw32 doesn't actually need to be mixed with gcc, so it's better
to separate them as they are supposed to be.
