Avoid errors about clang13-only options here, to ease transition

Avoid errors about clang13-only options here, to ease transition

As pointed out by deraadt@ we can do better than ask people to follow
instructions.  Note that you still need an up-to-date clang if you
update your kernel Makefiles, as those use clang13-only options.

Also -Werror doesn't bring much in binutils context.  It's an outdated
codebase with lots of warts, which frequently breaks due to -Werror
during clang updates.

"This is the right fix" deraadt@