Add a wrapper for llvm-config which provides flags from the current envrionment
instead of the values hardcoded into llvm-native at compile time. Inspiration
taken from the wrapper in meta-clang but I had to totally rewrite it as:
* the TARGET_* prefixes weren't in our environment
* meson uses --libs --ldflags XXX which didn't work
(From OE-Core rev: 193ee1973f613b72f7f99660aecf652b07652d18)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>