A64: Add and use a double register which holds the 0.0 value.

This patch also modify the crankshaft allocatable double registers because
we need this register to be callee saved to be efficient.

R=jochen@chromium.org

Review URL: https://codereview.chromium.org/190663009

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19803 ce2b1a6d-e550-0410-aec6-3dcde31c8c00