Migrate the compiler to LLVM toolchain and update config as per required and preferred config options by kconfigcheck. Update the APKBUILD to follow some more recent standards as well. Since the kernel image got too big (again) for msm8974, activate CONFIG_CC_OPTIMIZE_FOR_SIZE and CONFIG_KERNEL_XZ to get the kernel a bit smaller again to fit below the magical limit (see !5725). Part-of: <https://gitlab.postmarketos.org/postmarketOS/pmaports/-/merge_requests/7538>