Fix a Y2038 bug by replacing `Int32x32To64` with regular multiplication (#2471)

Fix a Y2038 bug by replacing `Int32x32To64` with regular multiplication (#2471)

`Int32x32To64` macro internally truncates the arguments to int32, while
`time_t` is 64-bit on most/all modern platforms. Therefore, usage of
this macro creates a Year 2038 bug.

I detailed this issue a while ago in a writeup, and spotted the same
issue in this repository when updating the list of affected
repositories:
<https://cookieplmonster.github.io/2022/02/17/year-2038-problem/>

A few more notes:
1. I changed all uses of `Int32x32To64` en masse, even though at least
one of them was technically OK and used with int32 parameters only. IMO
better safe than sorry.
2. This is untested, but it's a small enough change that I hope the CI
success is a good enough indicator.