This has been added in 0071a79532e8d664b734956a431d8c8c942cc25e to support TLS destructors. Return value of __cxa_thread_atexit is supposed to be the same as std::atexit - zero on success, non-zero on failure. Update the code to do just that (also be consistent with llvm-libc's existing atexit / at_quick_exit implementations).
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