arm64: Reduce the direct use of cpu_desc

[andrew]

Summary

To help moving to a dynamically allocated cpu_desc array reduce the
places we use it directly and create a pointer that is passed in to
functions that read it.

Sponsored by: Arm Ltd

[zachary.leaf_arm.com]

sys/arm64/arm64/identcpu.c

2453

Functionally, we used to compare everything with cpu_desc[0], now we compare with prev_desc - does that matter?

[andrew]

sys/arm64/arm64/identcpu.c

2453

In the base case where prev_desc is CPU0 the check is identical.

When prev_desc is not CPU0 we know that at this affinity level and all higher levels the value is identical so it doesn't matter which cpu we compare with. This is true because cpu_aff_levels is set to the affinity level where above it all CPUs have an identical affinity and below it some CPUs are different.