x86/ioapic.c: unify ioapic_retrigger_irq()

The 32 and 64-bit versions of ioapic_retrigger_irq() are identical
except the 64-bit one takes vector_lock.  vector_lock is defined and
used on 32-bit too, so just use a common ioapic_retrigger_irq().

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>