Revert "Remove V8_HOST_CAN_READ_UNALIGNED and its uses."

This reverts r23915.

TBR=machenbach@chromium.org

Review URL: https://codereview.chromium.org/569783003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23925 ce2b1a6d-e550-0410-aec6-3dcde31c8c00