[stable] core.cpuid: Fix div-by-zero on AMD CPUs with 256 (physical?) cores

[kinke]

See: https://en.wikipedia.org/wiki/CPUID#EAX=80000008h:_Virtual_and_Physical_address_Sizes

This might fix Issue 24254, although I'd expect the read value for that CPU to be 127 (physical cores minus 1), not the problematic 255.

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✗	24254	major	LDC crash on Epyc Bergamo

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "stable + dmd#15859"

[ibuclaw]

Seems reasonable to me.