[IA64] Initialize interrupts later (from init_IRQ())

Thomas Gleixner is cleaning up the generic irq code, and ia64 ran
into problems because it calls register_intr() before early_irq_init()
is called.  Move the call to acpi_boot_init() from setup_arch() to
init_IRQ().

As a bonus - moving the call later means we no longer need the
hacks in iosapic.c to switch between the bootmem and regular
allocator - we can just used kzalloc() for allocation.

Signed-off-by: Tony Luck <tony.luck@intel.com>