guix: pre time-machine bump changes (Windows)

Summary:
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

Test Plan: HOSTS="x86_64-w64-mingw32" contrib/guix/guix-build

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15485