[cpuinfo] Set CPUINFO_MOCK in MSan builds

cpuinfo is currently broken in MSan builds (see bug). Mock it until we
find a fix.

Bug: 331926164
Change-Id: I01e23fd7315388acec32e840c2068a1667a4602b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5417594


Owners-Override: Munira Tursunova <moonira@google.com>
Commit-Queue: Munira Tursunova <moonira@google.com>
Auto-Submit: Hans Wennborg <hans@chromium.org>
Reviewed-by: Munira Tursunova <moonira@google.com>
Cr-Commit-Position: refs/heads/main@{#1281764}