i#1569 AArch64: Implement dr_try_start.

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

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

core/arch/aarch64/aarch64.asm

@@ -352,9 +352,14 @@ GLOBAL_LABEL(memset:)
 
 #ifdef CLIENT_INTERFACE
 
+/* Xref x86.asm dr_try_start about calling dr_setjmp without a call frame.
+ *
+ * int dr_try_start(try_except_context_t *cxt) ;
+ */
         DECLARE_EXPORTED_FUNC(dr_try_start)
 GLOBAL_LABEL(dr_try_start:)
-        bl       GLOBAL_REF(unexpected_return) /* FIXME i#1569: NYI */
+        add      ARG1, ARG1, #TRY_CXT_SETJMP_OFFS
+        b        GLOBAL_REF(dr_setjmp)
         END_FUNC(dr_try_start)
 
 /* We save only the callee-save registers: X19-X30, (gap), SP, D8-D15.