Add mkspec for clang using libc++ on Linux.

libc++ is an alternative stdlib implementation for clang.
See http://libcxx.llvm.org/ for further details.
The library is enabled by adding -stdlib=libc++ to the command line.

Change-Id: I07d09cbb69b59b579d3754c99d717d2ac6d44d67
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>