[bug#66263] Updated patch series.

[Jean-Pierre De Jesus Diaz]

Hello,

I've sent an updated revision of the changes, now adding
the MinGW toolchain targets and also changing the Ergodox
and QMK firmwares to use #:target "avr" for cross-compilation,
so it removes make-avr-toolchain from `qmk` inputs as packages
should use #:target "avr" or in user profiles cases they can install
the toolchain from the packages in cross-toolchain.scm.

The QMK package warns about the firmware not compiling with
GCC 8+ but it seems to be working right now with the default
GCC used in Guix, so maybe it'd be to tie the package to the
default GCC version as IIRC there's no way to pass a custom base
GCC version when using #:target means of cross compilation.

I've also moved the toolchain procedure to cross-base.scm and
memoized it.

-- 
Jean-Pierre De Jesus DIAZ
Foundation Devices