arm64: prevent possible deadlock on SMP with FPU sharing

Let's consider CPU1 waiting on a spinlock already taken by CPU2.

It is possible for CPU2 to invoke the FPU and trigger an FPU exception
when the FPU context for CPU2 is not live on that CPU. If the FPU context
for the thread on CPU2 is still held in CPU1's FPU then an IPI is sent
to CPU1 asking to flush its FPU to memory.

But if CPU1 is spinning on a lock already taken by CPU2, it won't see
the pending IPI as IRQs are disabled. CPU2 won't get its FPU state
restored and won't complete the required work to release the lock.

Let's prevent this deadlock scenario by looking for pending FPU IPI from
the spinlock loop using the arch_spin_relax() hook.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>