Also moves CallStubN to be a private member of code-assembler.

BUG=v8:6949,v8:11074

Change-Id: I88a36819aead919cc4f4deff201925562fc9f74f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2527061Reviewed-by: Santiago Aboy Solanes <solanes@chromium.org>
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Auto-Submit: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71086}