Revert "Use constant length for memcpy on A64 simulator."

This reverts commit r20141 which broke the build with some GCC versions. The
reason behind that is GCC emits a warning about code which can never be reached,
but GCC can't figure that out.

The right approach would be using template specialization instead of these
runtime if/then/else cascades, which would be nicer from a SW engineering
perspective, too.

TBR=yangguo@chromium.org

Review URL: https://codereview.chromium.org/207703003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20142 ce2b1a6d-e550-0410-aec6-3dcde31c8c00