Remove V8_HOST_CAN_READ_UNALIGNED and its uses.

R=jkummerow@chromium.org
BUG=chromium:412967
LOG=Y

Review URL: https://codereview.chromium.org/568753002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23915 ce2b1a6d-e550-0410-aec6-3dcde31c8c00