Split out of #27897. This is some refactoring to the Windows Guix build
that facilitates bumping our Guix time-machine. Namely, avoiding
`package-with-extra-configure-variable`, which is non-functional in the
newer time-machine, see https://issues.guix.gnu.org/64436.
At the same time, consolidate our Windows GCC build into mingw-w64-base-gcc.
Rename `gcc-10-remap-guix-store.patch` to avoid changing it whenever GCC changes.
We move the old `building-on` inside `explicit-cross-configure`, so that
non-windows builds continue to work. Note that `explicit-cross-configure`
will be going away entirely (see #27897).
This is a backport of core#28294