breakage after changes related to gcc6 in Alpine

Recap of what happened:

• Alpine upgraded gcc from 6 to 8 a few weeks ago
• We learned that at least one device won't boot when the kernel is compiled with gcc8.
• We added a gcc6 package (and cross compilers: gcc6-armhf, gcc6-aarch64) and adjusted all existing kernel packages. I carefully made sure that all kernels compile with this version (tweaked everything until it did).
• Alpine added their own gcc6 package two days ago, with java support enabled
• It has a higher version than the our version, so pmbootstrap prefers it over our own aport
• linux pmaports don't compile with it anymore, as the binaries have slightly different names (the name collision with Alpine's gcc package that is GCC8 now, was solved differently) (#138 (closed))
• we adjusted the linux pmaports to work with Alpine's gcc6 (ee659a5b) and deleted our gcc6 (d56a2fac)
• All kernel pkgrels were bumped, which means all kernels must be rebuilt now.
• I did not compile all kernels before merging this change and now it turns out that it does not work with all kernels. My binary repo building scripts are stuck at linux-infocus-flatfish:

gcc6-armv6-alpine-linux-muslgnueabihf-ld: cannot find standby.o: No such file or directory

• (side story: gcc6 is not available for armhf/aarch64 yet, as Alpine's builders are stuck. pmbootstrap used to install that package, although it was not needed for our cross compiling. this was fixed in https://gitlab.com/postmarketOS/pmbootstrap/commit/977323cf6e6c10643f646ff435448a5ff8a27522 and https://gitlab.com/postmarketOS/pmbootstrap/commit/0e9644c0f9f5c0d1db674850c0fbdc25f94e5661. I've also tried to build Alpine's gcc6 for armhf and aarch64, but this was not feasible, see 9933a271).

I think the best way to resolve this, is going back to the postmarketOS specific gcc6 package that I created and tested with all kernels earlier. There is no need to update the gcc6 package ever, so I would just change pkgrel=9999 to make sure pmbootstrap will never use Alpine's gcc6 package.

TODO:

• locally restore the postmarketOS gcc6 aport and set pkgrel=9999
• locally bump all kernel pkgrels again and build all kernels with this gcc6 to make sure it is still working
• push the changes to pmaports master
• kick off building the packages
• building is done, everything is back to normal