Make the compiler emit visibility information for (undefined) references with

Make the compiler emit visibility information for (undefined) references with
non-default visibility.

See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=20218 for details.

This version comes from FreeBSD and has been made available under the GPLv2
license.  It has some additional bits thrown in from me to make it work in
mips64 too, and another bit to stop the C++ compiler to randomly emit
visibility information for C++ symbols that in the end aren't referenced.

ok guenther@