Consider adding nostdlib++ whenever a stdlib is explicitly indicated

[jwbee]

This toolchain adds enough flags to cause programs to be linked to libc++ and libc++abi, in which case the build will succeed with the -nostdlib++ flag. The flag will also prevent the eventuality that a program is linked to both libc++ and libstdc++. Consider adding it to the toolchain flags where appropriate.

A related safety improvement would be to add nostdinc++ whenever the libc++ header paths are being given as flags, to prevent searching for (and erroneously finding) the GNU c++ headers.