tcg: integrate upstream version

Change-Id: Ifcdebc2e7179fbc64b46a9150e6dae62f86eba3c
diff --git a/exec.c b/exec.c
index 285c950..f650a9e 100644
--- a/exec.c
+++ b/exec.c
@@ -521,6 +521,11 @@
 #if !defined(CONFIG_USER_ONLY)
     io_mem_init();
 #endif
+#if !defined(CONFIG_USER_ONLY) || !defined(CONFIG_USE_GUEST_BASE)
+    /* There's no guest base to take into account, so go ahead and
+       initialize the prologue now.  */
+    tcg_prologue_init(&tcg_ctx);
+#endif
 }
 
 #if defined(CPU_SAVE_VERSION) && !defined(CONFIG_USER_ONLY)