build: Check for std::atomic::exchange rather than std::atomic_exchange

Our usage of std::atomic is with it's own exchange function, not
std::atomic_exchange. So we should be looking specifically for that
function.

Additionally, -pthread and -lpthread have an effect on whether -latomic
will be needed, so the atomics check needs to use these flags as well.
This will make the flags in use better match what is actually used when
linking.

This removes the need for -latomic for riscv builds, which resolves a
guix cross architecture reproducibility issue.

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