[libc++] Mark __call_once_proxy as hidden and internal

We effectively never want to export that function, which is an
implementation detail of libc++. This was previously tried in
603715c and then reverted in 8335dd3 because it caused
linker warnings. These linker warnings should go away now that we
use internal_linkage instead of always_inline to implement per-TU
insulation.

Author: ldionne

libcxx/include/mutex

@@ -650,7 +650,7 @@ public:
 #endif
 
 template <class _Fp>
-void
+void _LIBCPP_INLINE_VISIBILITY
 __call_once_proxy(void* __vp)
 {
     __call_once_param<_Fp>* __p = static_cast<__call_once_param<_Fp>*>(__vp);