ARM64: Access has_pending_message_ correctly.

This fixes accesses in ARM and ARM64; the field is a bool, with size 1,
but we were accessing it with pointer-sized loads and stores.

BUG=
R=ulan@chromium.org

Review URL: https://codereview.chromium.org/230553004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20771 ce2b1a6d-e550-0410-aec6-3dcde31c8c00