When instantiating the wasm interpreter, pass the start address of the
global variables. This was nullptr before, leading to a crash if
debugging a program which accesses globals.
With test.

R=ahaas@chromium.org, titzer@chromium.org
BUG=v8:5822

Change-Id: I5f419790042ef9a00787df093a07e5e5835d55bd
Reviewed-on: https://chromium-review.googlesource.com/456219
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43948}