Store offset between fixed typed array base and data start in object

Store offset between fixed typed array base and data start in object

The layout of fixed typed array base is then capable of handling
external typed arrays as well. In a follow-up CL, I'll delete external
typed arrays, and use fixed typed array base instead

BUG=v8:3996
R=jarin@chromium.org,mstarzinger@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/1248483007

Cr-Commit-Position: refs/heads/master@{#29786}