Instead of building cross packages for the native architecture, add a
new get_cross_packages_arches() function that returns the desired
architectures no matter what the current native architecture is.
So gcc-x86_64 will always have arch="aarch64", and gcc-aarch64 will
always have arch="x86_64".
We can tweak the function in the future if e.g. it becomes viable to
cross compile from riscv64. Or even better, remove this and the cross
compile packages upstreamed into alpine (see pmb#551).
Link: https://lists.sr.ht/~postmarketos/pmbootstrap-devel/%3C20231126104823.3723-1-ollieparanoid@postmarketos.org%3E
62a05d4fbc