This is a reland of 9eba2d85.
The reland fixes a global state variable which was incompatible
with the --isolate flag in d8, which runs the same script in a
different isolate.

Original change's description:
> [fastcall] Add fast API testing facilities to d8
>
> This CL provides the minimum necessary functionality to expose fast API
> for testing in mjsunit, exposing the fast path for fuzzing. It exposes
> a d8.test.fast_c_api with an `add_all` method, which exercises primitive
> types. On x64, all integer and floating point types are supported. On
> other platforms currently only 32-bit integers are included in the test.
>
> Design doc:
> https://docs.google.com/document/d/1KUKPfXkSRZTA2gMwaWbpQKlYfw0C-T6AE3XzC4viHbo/
>
> Bug: chromium:1052746
> Change-Id: Icc824199a26dd2abd2b869f5483a39d38e4dce3e
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2749154
> Reviewed-by: Camillo Bruni <cbruni@chromium.org>
> Reviewed-by: Georg Neis <neis@chromium.org>
> Reviewed-by: Sathya Gunasekaran  <gsathya@chromium.org>
> Commit-Queue: Maya Lekova <mslekova@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#73670}

Bug: chromium:1052746
Change-Id: I33b265b97bf7c797eee7d4cce5066999358a8c66
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2790174Reviewed-by: Georg Neis <neis@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Sathya Gunasekaran  <gsathya@chromium.org>
Commit-Queue: Maya Lekova <mslekova@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73801}