Route sysctlbyname of "hw.model" to a utility function

"hw.model" is deprecated, and because there's already a utility
function to get the correct value, use it.

Bug: none
Low-Coverage-Reason: Altering code not mine, no existing tests.
Change-Id: Iebdca1fa0885743e1478956fc2c1580ed26a2038
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4833232
Reviewed-by: Yann Dago <ydago@chromium.org>
Auto-Submit: Avi Drissman <avi@chromium.org>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Yann Dago <ydago@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1191557}