gh-99248: [Enum] fix negative number infinite loop (GH-99256)

[Enum] fix negative number infinite loop

- _iter_bits_lsb() now raises a ValueError if a negative number
  is passed in

- verify() now skips checking negative numbers for named flags
(cherry picked from commit 0b4ffb08ccdc21fc07ce90d3f78b58a25e1af653)

Co-authored-by: Ethan Furman <ethan@stoneleaf.us>