We're weaning the core code off handing cpumask's around on-stack.
This introduces arch_send_call_function_ipi_mask().

We also take the chance to wean send_IPI_mask off the obsolescent
for_each_cpu_mask(): making it take the pointer seemed the most
natural way.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>