PATCH to memcpy visibility for middle-end/20297

[Jason Merrill]

The problem here was that builtin FUNCTION_DECLs built lazily were 
getting whatever the current visibility was at the time, which is wrong; 
they should always have default visibility.  The C++ front end already 
got this right (in build_library_fn_1), but some middle end code didn't yet.

Tested x86_64-pc-linux-gnu; applied to trunk, 4.1 and 4.0.