gnu: Python: Move arm-alignment.patch to a native-input + phase.

gnu: Python: Move arm-alignment.patch to a native-input + phase.

This follows up 67ca82e6ddc9f1c87fa8c033729a1f53b7c2b3ba which was ineffective
when running 'guix build --system=armhf-linux python' on e.g. x86_64-linux
because the conditional would always return the native system.

* gnu/packages/python.scm (python-3.8)[native-inputs]: Add "arm-alignment.patch".
[arguments]: Add phase to apply it.
[source](patches): Remove it.
* gnu/packages/commencement.scm (python-boot0): Prevent inheriting the phase.