We add clang as an alternative compiler, it sitll uses underlying
binutils and compiler runtime, it will only do the compilation and defer
link, to GNU ld/gold, to compile a component usign clang add

TOOLCHAIN_pn-<recipe-name> = "clang"
DEPENDS_append_pn-<recipe-name> = " clang-cross "

And OE will start using clang to compile <recipe-name>

Signed-off-by: Khem Raj <raj.khem@gmail.com>