Promote memory_order_consume to memory_order_acquire

[Hincoin]

The current use case of std::memory_order_consume in this project was intended to provide the appropriate synchronization in cases where a consumer spins on while( spsc_queue.empty() ) {}, and then attempts to use an element of the queue since the loop was broken out of, according to comments here. Consume semantics do not provide this guarantee according to the standard since there is no data dependency from the producer that can be carried to the consumer by doing a load-consume from the corresponding functions. What is needed is a load-acquire. Current compilers promote memory_order_consume to memory_order_acquire. Thus, this example appears to work simply due to the promotion from consume to acquire, but would fail to generate the right synchronization instructions on weaker architectures once memory_order_consume is implemented as intended. Therefore, the memory_order should be tightened to memory_order_acquire to guarantee visibility to the consumer.

[rigtorp]

👍 I agree, current implementation might break on non-x86_64 architectures in the future.