x86/ioapic.c: unify __mask_IO_APIC_irq()

The main difference between 32 and 64-bit __mask_IO_APIC_irq() does a
readback from the I/O APIC to synchronize it.

If there's a hardware requirement to do a readback sync after updating
an APIC register, then it will be a hardware requrement regardless of
whether the kernel is compiled 32 or 64-bit.

Unify __mask_IO_APIC_irq() using the 64-bit version which always syncs
with io_apic_sync().

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