A64: Fix compilation with clang

The size_ member is only used in debug mode, so we should only define
it in debug mode.

BUG=none
R=rodolph.perfetta@arm.com, ulan@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/171583004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19495 ce2b1a6d-e550-0410-aec6-3dcde31c8c00