clang-native: Add class to use clang as native compiler

clang-native: Add class to use clang as native compiler

Some recipes demand full clang/llvm builds e.g. chromium we need to use
clang as native toolchain. This class collects all needed bits to enable
OE built clang to provide the clang native toolchain

Setting

TOOLCHAIN_NATIVE = "clang"

in recipe will chose clang for native toolchain

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>