libsupc++: try cxa_thread_atexit_impl at runtime
g++.dg/tls/thread_local-order2.C fails when the toolchain is built for
a platform that lacks __cxa_thread_atexit_impl, even if the program is
built and run using that toolchain on a (later) platform that offers
__cxa_thread_atexit_impl.
This patch adds runtime testing for __cxa_thread_atexit_impl on
platforms that support weak symbols.
for libstdc++-v3/ChangeLog
* libsupc++/atexit_thread.cc [__GXX_WEAK__]: Add dynamic
detection of __cxa_thread_atexit_impl.