Implemented on ia32, x64, ARM. Stubbed out with UNIMPLEMENTED on MIPS.

BUG=none
TEST=unbox-double-arrays.js

Review URL: http://codereview.chromium.org/7307030

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@8637 ce2b1a6d-e550-0410-aec6-3dcde31c8c00