bug: Make BUG() always stop the machine

When !CONFIG_BUG and !HAVE_ARCH_BUG, define the generic BUG() as an
infinite loop rather than a no-op.  This avoids undefined behavior if
execution ever actually reaches BUG(), and avoids warnings about code
after BUG() (such as on non-void functions calling BUG() and then not
returning).

bloat-o-meter results:

  add/remove: 0/0 grow/shrink: 43/10 up/down: 235/-98 (137)
  function                             old     new   delta
  umount_collect                       119     138     +19
  notify_change                        306     324     +18
  xstate_enable_boot_cpu               252     269     +17
  kunmap                                54      70     +16
  balloon_page_dequeue                 112     126     +14
  mm_take_all_locks                    223     233     +10
  list_lru_walk_node                   143     152      +9
  vma_adjust                          1059    1067      +8
  pcpu_setup_first_chunk              1130    1138      +8
  mm_drop_all_l...