llvm: Add support for LLVM libc++ C++ Standard Library

LLVM toolchain provides its own C++ standard library called libc++.
This patch adds new LLVM_LIBCXX config which should be used to indicate
that libc++ is used.

Information about library can be found at https://libcxx.llvm.org



Signed-off-by: Patryk Duda <pdk@semihalf.com>