Before doing an indirect call, we always have to call {PrepareCall}.
Thus move this code out into platform-independent code. This also
slightly simplifies the API for {CallIndirect}.

R=ahaas@chromium.org

Bug: v8:6600
Change-Id: Id88ae5a9e73d1b6526131a5789205eda12b660de
Reviewed-on: https://chromium-review.googlesource.com/911369
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51230}