This was causing zig2.exe to crash during bootstrap, because there was an atomic
load of read-only memory, and the attempt to write to it as part of the (idempotent)
atomic exchange was invalid.
Aligned reads (of u32 / u64) are atomic on x86 / x64, so this is replaced with an
optimization-proof load (`__iso_volatile_load8*`) and a reordering barrier.
bc28454b43