libc++abi: make __cxa_call_unexpected visible

This may be invoked by the compiler, and needs to be made available so that the
users can reference it.

llvm-svn: 269255

libcxxabi/src/cxa_personality.cpp

@@ -1166,7 +1166,7 @@ __gxx_personality_v0(_Unwind_State state,
 
 
 __attribute__((noreturn))
-void
+_LIBCXXABI_FUNC_VIS void
 __cxa_call_unexpected(void* arg)
 {
     _Unwind_Exception* unwind_exception = static_cast<_Unwind_Exception*>(arg);