[Bug libstdc++/78968] conflict between gnu's __cxa_thread_atexit and LLVM's/FreeBSD's implementation

[kostikbel at ukr dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78968

--- Comment #18 from Konstantin Belousov <kostikbel at ukr dot net> ---
(In reply to Jonathan Wakely from comment #16)
> Trunk no longer defines __cxa_thread_atexit if it's found in libc. We might
> want to backport this to the gcc-5-branch and gcc-6-branch.
> 
> I will try to test this in a FreeBSD 11 VM some time soon.

I just built trunk gcc on FreeBSD HEAD.  Indeed, the newest gcc
does not export __cxa_thread_atexit from libstdc++.so.6 when configured
on a FreeBSD system which provides the symbol from libc.

I also added __cxa_thread_atexit_impl, which does help with older gcc versions.
I verified that libstdc++.so.6 provides __cxa_thread_atexit and expectedly
forwards the calls to libc:__cxa_thread_atexit_impl.

https://svnweb.freebsd.org/changeset/base/311651