Just keep things simple and zero the timestamp. It's not obvious that there's
any real benefit to adding complexity and harming performance by computing a
hash of the whole file for the timestamp value.

See:

* https://learn.microsoft.com/en-us/windows/win32/debug/pe-format#general-concepts
* https://learn.microsoft.com/en-us/windows/win32/debug/pe-format#debug-type

In the future, we should at least add an empty `IMAGE_DEBUG_TYPE_REPRO` entry to
the debug data directory for this case, but I didn't want to do too much surgery
right now since it's early days for COFF support in the self-hosted linker.