Now that builtins are embedded into the binary unconditionally, GC
visitation can be reduced. The interpreter dispatch table points
directly at embedded instruction starts. It is initialized once in
Isolate::Init, and its contents are immutable afterwards.
Visitation by GC is not needed.

Drive-by: Remove outdated comment on IsWasmRuntimeStub.

Bug: v8:7873
Change-Id: I14edc0beebb31c04f1429346b57ade9e8d838670
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1899773Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64814}