When using clang as a cross-compiler, we should not use system headers to do the compilation.
This CL adds support of a new warning flag -Wpoison-system-directories which emits warnings if --sysroot is set and headers from common host system location are used.
By default the warning is disabled.
The intention of the warning is to catch bad includes which are usually generated by third party build system not targeting cross-compilation. Such cases happen in Chrome OS when someone imports a new package or upgrade one to a newer version from upstream.