[QtScript/JavaScriptCore] Fix bugs and crashes due to return value register != 0 for native calls

Patch: http://gitorious.org/+qtwebkit-developers/qtwebkit/qtwebkit-qtscript-integration/commit/08e591816ff53954b8ea1b6c7a4da5587666ab5d

This doesn't look upstreamable. JSC must be using the "dst" value later for something.

Proposed action: Unapply the patch and find out which tests/apps crash. Store the flags somewhere else.