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