tsan: prevent the following false positive due to __cxa_atexit

WARNING: ThreadSanitizer: data race (pid=29103)
  Write of size 8 at 0x7d64003bbf00 by main thread:
    #0 free tsan_interceptors.cc:477
    #1 __run_exit_handlers <null>:0 (libc.so.6+0x000000050cb7)

  Previous write of size 8 at 0x7d64003bbf00 by thread T78 (mutexes: write M9896):
    #0 calloc tsan_interceptors.cc:449
    #1 ...

llvm-svn: 190989