Move NewStructInternal to header and templatize it to unroll
initialisation loop.

Bug: v8:11263
Change-Id: Iaaf2929c9a17b9195177b6afa7087b9b4ed6f0b4
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2821706Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Anton Bikineev <bikineev@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74088}