I recently tried to write an ebuild for an ObjC framework and always got errors when compiling. The command "ln -s /usr/bin/clang /usr/local/bin/x86_64-pc-linux-gnu-clang" fixed everything. (/usr/local/ because I expect the symlink from the package to appear in /usr/bin :]) I think that this symlink should be created when installing clang.
Didn't we have a bug for it already? Well, since OP needs that, I guess we can start doing it. We'll just have to make sure that no ebuilds in the tree start catching clang instead of preferred CC...
Cleaning old bugs, current ebuilds have x86_64-pc-linux-gnu-clang and friends in /usr/bin