Solution: prefer CXX11 atomics if they are available to compiler
intrinsics.
test_hwm_pubsub fails 50% of the times on PPC64 and RISC-V with an
apparent memory corruption of messages sent by the application thread
to the I/O thread when using compiler intrinsics.
Switching to CXX11 atomics makes the test reliable again. The
standard API should be preferred anyway, if available.