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_locks                    143     151      +8
  ns_capable                            55      62      +7
  anon_transport_class_unregister        8      15      +7
  srcu_init_notifier_head               35      41      +6
  shrink_dcache_for_umount             174     180      +6
  kunmap_high                           99     105      +6
  end_page_writeback                    43      49      +6
  do_exit                             1339    1345      +6
  __kfifo_dma_out_prepare_r             86      92      +6
  __kfifo_dma_in_prepare_r              90      96      +6
  fixup_user_fault                     120     125      +5
  repair_env_string                     73      77      +4
  read_cache_pages_invalidate_page      56      60      +4
  isolate_lru_pages.isra               142     146      +4
  do_notify_parent_cldstop             255     259      +4
  cpu_init                             370     374      +4
  utimes_common                        270     272      +2
  tasklet_hi_action                     91      93      +2
  tasklet_action                        91      93      +2
  set_pte_vaddr                         46      48      +2
  find_get_pages_tag                   202     204      +2
  early_iounmap                        185     187      +2
  __native_set_fixmap                   36      38      +2
  __get_user_pages                     822     824      +2
  __early_ioremap                      299     301      +2
  yield_task_stop                        1       2      +1
  tick_resume                           37      38      +1
  switched_to_stop                       1       2      +1
  switched_to_idle                       1       2      +1
  prio_changed_stop                      1       2      +1
  prio_changed_idle                      1       2      +1
  pm_qos_power_read                    111     112      +1
  arch_cpu_idle_dead                     1       2      +1
  __insert_vmap_area                   140     141      +1
  sys_renameat                         614     612      -2
  mm_fault_error                       297     295      -2
  SyS_renameat                         614     612      -2
  sys_linkat                           416     413      -3
  SyS_linkat                           416     413      -3
  chmod_common                         129     122      -7
  proc_cap_handler                     240     225     -15
  __schedule                           849     831     -18
  sys_madvise                         1077    1054     -23
  SyS_madvise                         1077    1054     -23

Signed-off-by: Josh Triplett <josh@joshtriplett.org>
Reported-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>