device/linux-*: make all kernels compile again

All kernels compile again, after the GCC 8 upgrade.

All 3.x kernels have been changed to use GCC 6 now, because we can't
say for sure that they boot with GCC 8 even if we made them compile
with that newer GCC. If someone wants to test a kernel for a newer
device which they can test, see the instructions on
<https://postmarketos.org/vendorkernel>.

The linux-sony-castor-windy 4.x kernel did not compile out of the box
with the latest GCC, so we set it to GCC 6 as well. This can probably
be fixed easily.

linux-teclast-x80-pro: removed Werror and updated to latest 4.14 kernel
from kernel.org, otherwise this would not compile anymore (tested with
both GCC 6 and 8, probably incompatibility with the latest binutils or
something).

This commit will take too long to compile in CI, so let's [skip ci].
I've compiled all kernels multiple times and fixed them up until all of
them were working again.

Related: #103