runtime: ensure memmove write pointer atomically on ARM64
If a pointer write is not atomic, if the GC is running
concurrently, it may observe a partially updated pointer, which
may point to unallocated or already dead memory. Most pointer
writes, like the store instructions generated by the compiler,
are already atomic. But we still need to be careful in places
like memmove. In memmove, we don't know which bits are pointers
(or too expensive to query), so we ensure that all aligned
pointer-sized units are written atomically.