Introduce tracing nops for aarch64.

Use the hlt instruction with a custom set of immediates to issue
tracing commands. This mirrors the behaviour of the ARM FVP Tarmac
trace plugin, which uses the hlt instruction with configurable
immediate values to start and stop tracing.

Introduce the following hlt immediates:
 - 0xff00: start instruction tracing
 - 0xff01: stop instruction tracing
 - 0xff02: start user-mode instruction tracing

target/arm/translate-a64.c

@@ -2697,7 +2697,32 @@ static void disas_exc(DisasContext *s, uint32_t insn)
 #endif
             gen_exception_internal_insn(s, s->pc_curr, EXCP_SEMIHOST);
         } else {
+#ifdef CONFIG_TCG_LOG_INSTR
+            TCGv tpc = tcg_const_tl(s->base.pc_next);
+            switch (imm16) {
+            case 0xff00:
+                gen_helper_qemu_log_instr_start(cpu_env, tpc);
+                s->base.is_jmp = DISAS_EXIT;
+                break;
+            case 0xff01:
+                gen_helper_qemu_log_instr_stop(cpu_env, tpc);
+                s->base.is_jmp = DISAS_EXIT;
+                break;
+            case 0xff02:
+                gen_helper_qemu_log_instr_user_start(cpu_env, tpc);
+                s->base.is_jmp = DISAS_EXIT;
+                break;
+            default:
+                unsupported_encoding(s, insn);
+            }
+            tcg_temp_free(tpc);
+
+            if (s->base.is_jmp != DISAS_NEXT) {
+                gen_a64_set_pc_im(s->base.pc_next);
+            }
+#else
             unsupported_encoding(s, insn);
+#endif
         }
         break;
     case 5: