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

[iains at gcc dot gnu.org]

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

--- Comment #9 from Iain Sandoe <iains at gcc dot gnu.org> ---
(In reply to Iain Sandoe from comment #8)
> (In reply to Iain Sandoe from comment #7)
> > it's been broken a looooong time - even apple-gcc-4.2.1 is broken
> 
> actually, that's not true; it puts the symbol in common to merge this.

so the culprit is:
gcc/config/darwin.h:
#define TARGET_WEAK_NOT_IN_ARCHIVE_TOC 1

.. this has been there for a long time.

So, now I need to audit what versions of ld64 really don't care about this.

Setting this to 0 (with recent as/ld64), makes the testcase complete without
throwing. Bootstrapping and checking on a few systems.