Further adjustments. It would have been better if I tried to build
it first, sorry for that. Another patch is coming in the pmbootstrap
repo, which makes it work even without the gcc6 pmaport.
Copy Alpine's gcc6 aport to temp. They don't have it built for aarch64
and armhf at the moment. Due to dependency checks, this means we can't
build the kernels that need gcc6, even when cross compiling with
gcc6-armhf etc. See #138 for details.