Add a "relaxed" memory order (#8984)

The memory order is not the C++ relaxed memory order, but rather the
slightly stronger load-store ordered memory order, which prevents
out-of-thin-air values by forbidding earlier atomic loads from being
reordered past later atomic stores. This makes it impossible to
construct the self-justifying causality cycle that gives rise to
out-of-thin-air values. The name of the memory order in both the
implementation and the text format is likely to change in the future to
become more descriptive.