declare assembly-only symbols with hidden visibility

In a future world where librrpreload is compiled with -fPIC, we don't
want to be calling these functions through PLT stubs (and then possibly
invoking the dynamic linker).  Make them hidden instead, which is
marginally more efficient even with position-dependent code.

I wish this was sufficient for turning on `-fPIC`, but I still see odd failures locally.  I'm not completely sure whether that's the VM's fault, or some subtlety of the code still to be dug out.