{NativeModule::GetJumpTableOffset} is totally independent of the
{NativeModule}, hence should be an independent function. It's now
defined in wasm-module.h, and works only on the WasmModule and the
function index.
{NativeModule::GetCallTargetForFunction} only has a single caller, which
is the wasm instance object, which already has direct access to the jump
table start. Hence we can just add the jump table offset there, and do
not need another helper method. This also makes it more clear that we
are returning a jump table slot.

R=jkummerow@chromium.org

Change-Id: If2e4eb4b3622df08ba905bd10783199bbb59d50a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3781348Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81970}