A64: Use appropriate return types for the simulator fpcr and nzcv members.

R=ulan@chromium.org

Review URL: https://codereview.chromium.org/203343003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20043 ce2b1a6d-e550-0410-aec6-3dcde31c8c00