lib/ubsan: don't serialize UBSAN report

At the moment, UBSAN report will be serialized using a spin_lock().  On
RT-systems, spinlocks are turned to rt_spin_lock and may sleep.  This
will result to the following splat if the undefined behavior is in a
context that can sleep:

  BUG: sleeping function called from invalid context at /src/linux/kernel/locking/rtmutex.c:968
  in_atomic(): 1, irqs_disabled(): 128, pid: 3447, name: make
  1 lock held by make/3447:
   #0: 000000009a966332 (&mm->mmap_sem){++++}, at: do_page_fault+0x140/0x4f8
  irq event stamp: 6284
  hardirqs last  enabled at (6283): [<ffff000011326520>] _raw_spin_unlock_irqrestore+0x90/0xa0
  hardirqs last disabled at (6284): [<ffff0000113262b0>] _raw_spin_lock_irqsave+0x30/0x78
  softirqs last  enabled at (2430): [<ffff000010088ef8>] fpsimd_restore_current_state+0x60/0xe8
  softirqs last disabled at (2427): [<ffff000010088ec0>] fpsimd_restore_current_state+0x28/0xe8
  Preemption disabled at:
  [<ffff000011324a4c>] rt_mutex_futex_unlock+0x4c/0xb0...