rust: Use libc++ runtime when using clang with llvm runtime

rust: Use libc++ runtime when using clang with llvm runtime

meta-clang has options when it comes to C++ runtime, default is to use
gnu runtime, other options are llvm runtime and android runtime. This
patch helps when a distro is using llvm runtime for C/C++ runtime. It
informs the rust build system about right C++ runtime to configure for
when such a setting is used.

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