Adding layering check toolchain features

[beckerhe]

This adds support for the layering_check feature to the C++ toolchains in this repo and prepares CUDA targets for compliance with the layering check.

1. It defines the layering_check and related features.
2. It extends the x86_64 toolchain definitions by Clang C++ modules header mapping files (crosstool.cppmap). This mapping file declares the system headers for each toolchain.
3. It changes the @local_config_cuda//cuda:headers such that it directly exports all the headers from the subtargets - which is needed for compliance with the layering check.