cpumask: remove x86 cpumask_t uses.

Impact: cleanup

We are removing cpumask_t in favour of struct cpumask: mainly as a
marker of what code is now CONFIG_CPUMASK_OFFSTACK-safe.

The only non-trivial change here is vector_allocation_domain():
explicitly clear the mask and set the first word, rather than using
assignment.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>