Mutable globals are stored in the instance differently, so they must be
exported differently as well.

Also fix a counting bug that occurred when a module contained a global
and an imported mutable global (CalculateGlobalOffsets is called
twice).

Bug: v8:7625
Change-Id: I1cd7ef5d6ff7cb7e09239035f89d7b36d0436063
Reviewed-on: https://chromium-review.googlesource.com/1096673
Commit-Queue: Ben Smith <binji@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53681}