i#1569 AArch64: Implement dr_try_start.
i#1569 AArch64: Implement dr_try_start.

This fixes the tests client.exception and tool.drltrace.

Review-URL: https://codereview.appspot.com/302160043

AmesianX/dynamorio,AmesianX/dynamorio,AmesianX/dynamorio,AmesianX/dynamorio,AmesianX/dynamorio