Use `u64` for address/pointer-sized integer types.
Use `u64` for address/pointer-sized integer types.

If cross-compiling, we may not be able to depend on the size of an
integer on the target lining up with the size of an integer where we are
compiling, so it's safer to use the largest integer size that we know is
available.

quantheory/probe-c-api,quantheory/probe-c-api