bhyve: fix CPUID L3 Cache Size reporting for AMD/SVM

[kib]

Summary

Adjust leaf 0x8000_001D %ecx 3 on AMD (L3 cache params).
- Report cache as 1-way associative.  Glibc does not believe that there
  are fully associative L3 caches, ignoring the leaf and falling back to
  legacy way of reading cache params.
- Do not report 4095 logical CPUs per L3 cache, report the true total
  number of emulated CPUs.  The insanely large value tricked some
  version of glibc to overflow 32bit calculation of the L3 cache size,
  as reported in the PR.

Also, for leaf 0x8000_0008, do not clip ApicIdSize to zero if less than
4.  This effectively falls back to legacy.

PR:     279901

[meta]

As I commented on Bugzilla, this broke EL8s.
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=279901#c53

[kib]

Revert back to 1-way assoc cache.

[mp]

Thanks for the patch and update. LGTM!

[kevans]

LGTM, thanks!

[meta]

Let me test it too, within a day.