accel/tcg: Introduce tlb_read_idx

Instead of playing with offsetof in various places, use
MMUAccessType to index an array.  This is easily defined
instead of the previous dummy padding array in the union.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

include/exec/cpu-defs.h

@@ -111,8 +111,11 @@ typedef struct CPUTLBEntry {
                use the corresponding iotlb value.  */
             uintptr_t addend;
         };
-        /* padding to get a power of two size */
-        uint8_t dummy[1 << CPU_TLB_ENTRY_BITS];
+        /*
+         * Padding to get a power of two size, as well as index
+         * access to addr_{read,write,code}.
+         */
+        target_ulong addr_idx[(1 << CPU_TLB_ENTRY_BITS) / TARGET_LONG_SIZE];
     };
 } CPUTLBEntry;