Declare RTS-private prototypes with __attribute__((visibility("hidden")))

Declare RTS-private prototypes with __attribute__((visibility("hidden")))
This has no effect with static libraries, but when the RTS is in a
shared library it does two things:

 - it prevents the function from being exposed by the shared library

 - internal calls to the function can use the faster non-PLT calls,
   because the function cannot be overriden at link time.