A64 support for DoubleToIStub (truncating).

Added support for truncating DoubleToIStub and reorganize the macro-assembler
dToI operations to do the fast-path inline and the slow path by calling the
stub.

This a port essentially a port of https://codereview.chromium.org/23129003/.

R=jacob.bramley@arm.com, ulan@chromium.org

Review URL: https://codereview.chromium.org/160423002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19414 ce2b1a6d-e550-0410-aec6-3dcde31c8c00