kernel - Refactor cpumask_t to extend cpus past 64, part 2/2

* Expand SMP_MAXCPU from 64 to 256 (64-bit only)

* Expand cpumask_t from 64 to 256 bits

* Refactor the C macros and the assembly code.

* Add misc cpu_pauses()s and do a bit of work on the boot sequencing.