py: Added optimised support for 3-argument calls to builtin.pow()

Updated modbuiltin.c to add conditional support for 3-arg calls to
pow() using MICROPY_PY_BUILTINS_POW3 config parameter. Added support in
objint_mpz.c for for optimised implementation.
diff --git a/py/mpz.c b/py/mpz.c
index 6477c3f..230eb92 100644
--- a/py/mpz.c
+++ b/py/mpz.c
@@ -1395,9 +1395,6 @@
     mpz_free(n);
 }
 
-#if 0
-these functions are unused
-
 /* computes dest = (lhs ** rhs) % mod
    can have dest, lhs, rhs the same; mod can't be the same as dest
 */
@@ -1436,6 +1433,9 @@
     mpz_free(n);
 }
 
+#if 0
+these functions are unused
+
 /* computes gcd(z1, z2)
    based on Knuth's modified gcd algorithm (I think?)
    gcd(z1, z2) >= 0