device/linux-*: use GCC-6 for all 3.x kernels

...and also for the 4.x kernels of linux-teclast-x80-pro and
linux-sony-castor-windy. These did not compile out of the box with GCC 8
either (although that can probably be fixed easily).

linux-teclast-x80-pro: also remove Werror, otherwise this won't compile
anymore, even with GCC 6.

This commit will take too long to compile in CI, so let's [skip ci].

Related: #103 and <https://postmarketos.org/vendorkernel>