Consider adding nostdlib++ whenever a stdlib is explicitly indicated

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.

Start by locating the toolchain flags that add libc++ and libc++abi, then identify where a standard library or libc++ header paths are explicitly selected. Check the existing toolchain variants and determine which should receive -nostdlib++ and -nostdinc++; done means the relevant configurations avoid mixing libc++ with libstdc++ or GNU C++ headers.

Written by the indexing model from the issue text.