This is to go with toolchain_build changes which make LLVM cmake
also use libc++:
https://codereview.chromium.org/978963002/

May help with the memory sanitizer build, which wants most code
to be built with memory sanitizer (e.g., make a special build of libc++).

BUG= https://code.google.com/p/nativeclient/issues/detail?id=4119
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/1074253002