interp: don't ignore array indices for untyped objects

This fixes https://github.com/tinygo-org/tinygo/issues/1884.
My original plan to fix this was much more complicated, but then I
realized that the output type doesn't matter anyway and I can simply
cast the type to an *i8 and perform a GEP on that pointer.

testdata/init.txt

@@ -7,6 +7,8 @@ v4: 0 true
 v5: 0 false
 v6: false
 v7: 3 foo
+v8: barba
+v9: 3 2 3 4
 3
 3
 5