Fix stack bottom initialization for non-main modules.

Because PreMain() was now called before the new stack bottom
initialization, it still relied on the old version of initStackBottom(),
which may not handle the top few words of the stack correctly. This
patch also sets the stack bottom in PreMain() using the new approach.
Merge pull request #1481 from rbehrends/fix-stackscan2

Fix stack bottom initialization for non-main modules.