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

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