Implement truncation of double and tagged values when assigning to an element of a typed arrays in order to avoid depots.

BUG=1313
TEST=test/mjsunit/external-array.js

Review URL: http://codereview.chromium.org/6961019

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@8077 ce2b1a6d-e550-0410-aec6-3dcde31c8c00