The platform-specific simulators are only included on the respective
hardware, so no need to guard against other platforms.
Also, unify where the USE_SIMULATOR macro is evaluated. Make it cover
the whole implementation file.
Also, make sure to include the respective header file first, as
suggested by the style guide, and fix up includes in general.

R=ishell@chromium.org

Bug: v8:8562, v8:7490
Change-Id: Ifc6a87030364eab1e6a6fcbda008607ccd1c1f2f
Reviewed-on: https://chromium-review.googlesource.com/c/1384090Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58365}