tcg: Define tcg_insn_unit for code pointers

To be defined by the tcg backend based on the elemental unit of the ISA.
During the transition, allow TCG_TARGET_INSN_UNIT_SIZE to be undefined,
which allows us to default tcg_insn_unit to the current uint8_t.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <rth@twiddle.net>
diff --git a/include/exec/exec-all.h b/include/exec/exec-all.h
index 1c49a21..0766e24 100644
--- a/include/exec/exec-all.h
+++ b/include/exec/exec-all.h
@@ -145,7 +145,7 @@
 #define CF_COUNT_MASK  0x7fff
 #define CF_LAST_IO     0x8000 /* Last insn may be an IO access.  */
 
-    uint8_t *tc_ptr;    /* pointer to the translated code */
+    void *tc_ptr;    /* pointer to the translated code */
     /* next matching tb for physical address. */
     struct TranslationBlock *phys_hash_next;
     /* first and second physical page containing code. The lower bit