native cross: only require native arch depends

Do not try to build and install dependencies for the package's
architecture when compiling in the "native" mode. That mode is
described here in detail:
https://wiki.postmarketos.org/wiki/Build_internals#Cross-compile_types

This makes it possible to cross compile kernels again, which need to
be built with GCC6. We have switched to Alpine's GCC6 package, but it is
not available for armhf/aarch64 on edge yet, because Alpine's build bots
are stuck (right now armhf is not even listed):
http://build.alpinelinux.org/

Huge thanks to ryang2478/Decatf for posting this patch in pmaports#138!