This refactors the list of redirections of runtime call targets that
simulators maintain to be process-wide (as opposed to be per Isolate).
Such redirections are used for static C++ call targets which themselves
are process-wide, which makes this model a closer fit. Access is already
properly synchronized via a mutex.

Along the way this also introduces the {SimulatorBase} class as a common
base class for all simulator implementations.

R=clemensh@chromium.org

Change-Id: Iae8602c44b1b34cb916dde2b22c9403b0496b3d4
Reviewed-on: https://chromium-review.googlesource.com/823966
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50074}