cmake.bbclass: Enhance to emit a native toolchain cmake file

cmake.bbclass: Enhance to emit a native toolchain cmake file

Some components e.g. clang and its runtime needs the native toolchain
defines especially when doing nativesdk builds it uses
CROSS_TOOLCHAIN_FLAGS_NATIVE to pass native toolchain file, which is
then used to build native llvm tools needed during nativesdk builds

Moreover this would enable using OE built native toolchain e.g.
clang-native to build cmake based packages. e.g. libcxx and other llvm
runtime plus it can be used for build native packages which require clang

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