tcg: Introduce tcg_splitwx_to_{rx,rw}

Add two helper functions, using a global variable to hold
the displacement.  The displacement is currently always 0,
so no change in behaviour.

Begin using the functions in tcg common code only.

Reviewed-by: Joelle van Dyne <j@getutm.app>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

include/exec/exec-all.h

@@ -448,7 +448,7 @@ int probe_access_flags(CPUArchState *env, target_ulong addr,
  * Note: the address of search data can be obtained by adding @size to @ptr.
  */
 struct tb_tc {
-    void *ptr;    /* pointer to the translated code */
+    const void *ptr;    /* pointer to the translated code */
     size_t size;
 };