[Bug c++/87380] Explicit instantations should use weak symbols on darwin

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87380

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
On GNU/Linux the symbol in the shared library is a global unique symbol:

$ nm --defined-only -g liblib.so  | grep member
000000000020101c u _ZN1AIiE6memberE

It seems that we need to make it weak to ensure that's true on darwin.