Avoid comparing std::memory_order_relaxed with 0

Starting in C++20, std::memory_order becomes a scoped enum class, so
it cannot be compared to an integer, which happens when ASM_DEFINE
tries to detect if std::memory_order_relaxed is negative. Cast the
enumerator to its underlying type.

Bug: 311052584
Test: treehugger
Change-Id: Ic217c54628f6b13a86dcb83363690a87f0fc40e3