Possible data race in src/pthread/once_atomic.cpp

Possible data race is found when running test util_ipc_reliable_mq.cpp in boostorg/log. See https://github.com/boostorg/log/issues/162 for more details.
Valgrind shows the following errors for this test:
```
# Run command: valgrind --tool=helgrind binary_path
==537225== Possible data race during read of size 4 at 0x4FE76B8 by thread #3
==537225== Locks held: none
==537225== at 0x4FD453C: load (core_ops_gcc_atomic.hpp:88)
==537225== by 0x4FD453C: load (atomic_impl.hpp:338)
==537225== by 0x4FD453C: boost::thread_detail::enter_once_region(boost::once_flag&) (once_atomic.cpp:39)
==537225== by 0x4FBFED8: void boost::call_once(boost::once_flag&, void (&)()) (once_atomic.hpp:123)
==537225== by 0x4FBD17E: boost::detail::set_current_thread_data(boost::detail::thread_data_base*) (thread.cpp:160)
==537225== by 0x4FBD346: boost::(anonymous namespace)::thread_proxy(void*) (thread.cpp:174)
==537225== by 0x4C37EEE: mythread_wrapper (hg_intercepts.c:387)
==537225== by 0x5E4D2DD: start_thread (in /usr/lib64/libpthread-2.28.so)
==537225== by 0x6160E82: clone (in /usr/lib64/libc-2.28.so)
==537225==
==537225== This conflicts with a previous write of size 4 by thread #2
==537225== Locks held: 1, at address 0x4FE77B0
==537225== at 0x4FD53F5: store (core_ops_gcc_atomic.hpp:76)
==537225== by 0x4FD53F5: store (atomic_impl.hpp:330)
==537225== by 0x4FD53F5: boost::thread_detail::commit_once_region(boost::once_flag&) (once_atomic.cpp:74)
==537225== by 0x4FBFF66: void boost::call_once(boost::once_flag&, void (&)()) (once_atomic.hpp:138)
==537225== by 0x4FBD17E: boost::detail::set_current_thread_data(boost::detail::thread_data_base*) (thread.cpp:160)
==537225== by 0x4FBD346: boost::(anonymous namespace)::thread_proxy(void*) (thread.cpp:174)
==537225== by 0x4C37EEE: mythread_wrapper (hg_intercepts.c:387)
==537225== by 0x5E4D2DD: start_thread (in /usr/lib64/libpthread-2.28.so)
==537225== by 0x6160E82: clone (in /usr/lib64/libc-2.28.so)
==537225== Address 0x4fe76b8 is 0 bytes inside data symbol "_ZN5boost6detail12_GLOBAL__N_128current_thread_tls_init_flagE"
==537225==
==537225== ----------------------------------------------------------------
==537225==
==537225== Possible data race during read of size 4 at 0x4FE76BC by thread #3
==537225== Locks held: none
==537225== at 0x4FBD17F: boost::detail::set_current_thread_data(boost::detail::thread_data_base*) (thread.cpp:161)
==537225== by 0x4FBD346: boost::(anonymous namespace)::thread_proxy(void*) (thread.cpp:174)
==537225== by 0x4C37EEE: mythread_wrapper (hg_intercepts.c:387)
==537225== by 0x5E4D2DD: start_thread (in /usr/lib64/libpthread-2.28.so)
==537225== by 0x6160E82: clone (in /usr/lib64/libc-2.28.so)
==537225==
==537225== This conflicts with a previous write of size 4 by thread #2
==537225== Locks held: none
==537225== at 0x5E5476F: pthread_key_create (in /usr/lib64/libpthread-2.28.so)
==537225== by 0x4FC9F36: _ZN5boost6detail6invokeIPFvvEJEEEDTclclsr5boostE7forwardIT_Efp_Espclsr5boostE7forwardIT0_Efp0_EEEOS4_DpOS5_ (invoke.hpp:133)
==537225== by 0x4FBFF10: void boost::call_once(boost::once_flag&, void (&)()) (once_atomic.hpp:127)
==537225== by 0x4FBD17E: boost::detail::set_current_thread_data(boost::detail::thread_data_base*) (thread.cpp:160)
==537225== by 0x4FBD346: boost::(anonymous namespace)::thread_proxy(void*) (thread.cpp:174)
==537225== by 0x4C37EEE: mythread_wrapper (hg_intercepts.c:387)
==537225== by 0x5E4D2DD: start_thread (in /usr/lib64/libpthread-2.28.so)
==537225== by 0x6160E82: clone (in /usr/lib64/libc-2.28.so)
==537225== Address 0x4fe76bc is 0 bytes inside data symbol "_ZN5boost6detail12_GLOBAL__N_122current_thread_tls_keyE"
```