build-system: linux-module: Fix cross compilation.

build-system: linux-module: Fix cross compilation.

* guix/build-system/linux-module.scm (default-kmod, default-gcc): Delete
procedures.
(system->arch): New procedure.
(make-linux-module-builder)[native-inputs]: Move linux...
[inputs]: ...to here.
(linux-module-build-cross): New procedure.
(linux-module-build): Add TARGET.  Pass TARGET and ARCH to build side.
(lower): Allow cross-compilation.  Move "linux" and "linux-module-builder"
to host-inputs.  Add target-inputs.  Call linux-module-build-cross if
TARGET is set, linux-module-build otherwise.
* guix/build/linux-module-build-system.scm (configure): Add ARCH argument.
(linux-module-build): Adjust comment.

Signed-off-by: Danny Milosavljevic <dannym@scratchpost.org>