src: simplify loop arithmetic in `GetCPUInfo`

Cache the repeated operations and reuse; potentially generating
efficient code in some platforms, and improving readability.

PR-URL: https://github.com/nodejs/node/pull/26183


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: James M Snell <jasnell@gmail.com>