Doesn't build without target_has_atomic="64"

[vasi]

On some platforms, such as 32-bit PowerPC/MIPS, atomic loads/stores of 64-bit values are not possible. Under these conditions, Nucleo (and therefore Helix) will fail to build, due to use of AtomicU64.

Would anything terrible happen if we fell back to AtomicU32 on those platforms?