Fix errors when converting negative numbers to uint64 representations

- In the previous implementation, we're receiving warnings when the
  input val is negative. That warning now becomes an ERROR starting from
numpy 2.0
- Updated the cast usage to achive the old behavior (cast overflows) as
  suggested by the warning.

Signed-off-by: Jerry Ge <jerry.ge@arm.com>
Change-Id: I21cb8a0c7dbac898ff5b06b9ead62070a9edd2b3