kbuild: LLVMLinux: Adapt warnings for compilation with clang

When compiling kernel with clang, disable warnings which are too noisy, and
add the clang flag catch-undefined-behavior.

Signed-off-by: Jan-Simon Möller <dl9pf@gmx.de>
Signed-off-by: Behan Webster <behanw@converseincode.com>
Signed-off-by: Mark Charlebois <mcharleb@gmail.com>
Cc: PaX Team <pageexec@freemail.hu>