clang doesn't propagate attributes like "asm labels" and "visibility(hidden)"

clang doesn't propagate attributes like "asm labels" and "visibility(hidden)"
to builtins like mem{set,cpy,move} and __stack_smash_handler.  So, when
building with clang, instead mark those as protected visibility to get rid
of the PLT relocations.  We can't take the address of them then, but that's
ok: it's a build-time error not a run-time error.

ok kettenis@