arm64: Malloc the cpu_desc array

[andrew]

Summary

We only need this during boot. Allocate the array before starting CPUs
to reduce the memory usage.

Sponsored by: Arm Ltd

[zachary.leaf_arm.com]

sys/arm64/arm64/identcpu.c

1862

It's the same thing, but mp_ncpus-1 seems clearer for what we're passing here as the first parameter i.e. malloc_array(size_t nmemb...)

[andrew]

sys/arm64/arm64/identcpu.c

1862

I decided on mp_maxid as it is allocating an array indexed by cpu id. If for some reason we had a sparse cpu id space then mp_ncpus - 1 may be too small.