libcxx,compiler-rt-sanitizers: Add cflags to build with clang compiler

libcxx,compiler-rt-sanitizers: Add cflags to build with clang compiler

We are using clang-native to build the native versions of these recipes
but we are missing the needed compiler flags to let it c/c++ runtime
appropriately. This also ensures that meta-clang does not have to worry
about the compiler settings for nativesdk/native pieces separately.

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