On 32-bit x86, both of these types have an alignment of 32 bits, which means
that atomic operations involving these types require a runtime call. We can
avoid the overhead of the runtime call, as well as a dependency on libatomic,
by aligning both types to 64 bits.
As far as I can tell these types are not part of the library's public
interface, so this change should be fine from an ABI perspective.