This CL introduces a new fast_api_call_target field on the isolate,
which is set by Turbofan before making the fast call. It then uses
the field when creating a stack sample and stores it in the existing
external_callback_entry used for regular API callbacks. The CL also
adds a cctest with simple usage scenario and introduces a minor
refactoring in test-api.cc.

Design doc:
https://docs.google.com/document/d/1r32qlPzGz0P7nieisJ5h2qfSnWOs40Cigt0LXPipejE/edit

Bug: chromium:1052746
Change-Id: I2dab1bc395ccab0c14088f7c354fb52b08df8d32
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2488683
Commit-Queue: Maya Lekova <mslekova@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71254}