MinGW-w64: Early create sysroot/mingw top dir

.. it must exist before building GCC as otherwise
the build will deliberately fail.

Left some WIP stuff in regarding the name of this
dir, as we would like to name them:

CT_MULTILIB=y -> mingw
CT_ARCH_64=y  -> mingw64
else          -> mingw32

.. but GCC doesn't support this at present.