llvm: Add llvm-config wrapper to improve flags handling

llvm: Add llvm-config wrapper to improve flags handling

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

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>