A64: Hardwire the decoder and the simulator

If one of --trace-sim --debug-sim or --log-instruction-stats flags is
given, we use the decoder dispatcher instead.

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

Review URL: https://codereview.chromium.org/177533023

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19564 ce2b1a6d-e550-0410-aec6-3dcde31c8c00