Match declarations of at_quick_exit and __cxa_atexit symbols for libc++ and C++17
Match declarations of at_quick_exit and __cxa_atexit symbols for libc++ and C++17

On Linux, the same adjustments are needed as for libstdc++ and libc++,
so turn the condition into just #ifdef __linux__. However, since the
__cxa_atexit function is declared as extern "C" in libc++, we cannot
add "noexcept" to it, as is done for libstdc++.

root-mirror/cling,root-mirror/cling,karies/cling,karies/cling,karies/cling,karies/cling,root-mirror/cling,root-mirror/cling,root-mirror/cling,karies/cling,root-mirror/cling,karies/cling