Now that only ppc lacks support, state that explicitly s.t. it's clear
which platforms are still missing.

Bug: v8:11420
Change-Id: I22a91270aaa389fc27ce5c7a3f8e8b9c44f01eb4
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3497544
Auto-Submit: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79340}