Kbuild: LLVMLinux: Adapt warnings for compilation with clang

When compiling kernel with clang, disable warnings which are too noisy.

Author:  PaX Team <pageexec@freemail.hu>
ML-Post: http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20120507/142707.html
URL:     http://llvm.linuxfoundation.org
Merge:   Jan-Simon Möller <dl9pf@gmx.de>
Signed-off-by: Jan-Simon Möller <dl9pf@gmx.de>
Clang does not know this flag. Make it conditional.

Signed-off-by: Jan-Simon Möller <dl9pf@gmx.de>

Conflicts:
	arch/arm/boot/compressed/Makefile