kbuild: LLVMLinux: Add Kbuild support for building kernel with Clang

kbuild: LLVMLinux: Add Kbuild support for building kernel with Clang

Add support to toplevel Makefile for compiling with clang, both for
HOSTCC and CC. Use cc-option to prevent gcc option from breaking clang, and
from clang options from breaking gcc.

Clang 3.4 semantics are the same as gcc semantics for unsupported flags. For
unsupported warnings clang 3.4 returns true but shows a warning and gcc shows
a warning and returns false.

Signed-off-by: Behan Webster <[email protected]>
Signed-off-by: Jan-Simon Möller <[email protected]>
Signed-off-by: Mark Charlebois <[email protected]>
Cc: PaX Team <[email protected]>