This adds a temporary mechanism for isolate-independent calls and
jumps.  The problem was that - as ia32 doesn't have a scratch register
- Call and Jump cannot call through a register. This CL adds a
so-called virtual register (= a pointer-sized field) on IsolateData.

The virtual register can be removed once pc-relative calls are
implemented and all builtins have been embedded.

Bug: v8:6666
Change-Id: I1f9d8a25643fad0b3919dd813dbe219d20fcc6bc
Reviewed-on: https://chromium-review.googlesource.com/c/1282991
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56693}