change attributes in role Native from int8 to int

This blew up in module Box2D which is about bindings to a C++ library. The
changed attributes point to not allocated memory. The program crash was
triggered by a method "Box2D::Body.CreateFixture" which heavily involves
traits. So perhaps the error only shows up when a certain level of traits or
mixins are involved.
See https://irclog.perlgeek.de/moarvm/2018-04-11#i_16035620

lib/NativeCall.pm6

@@ -266,8 +266,8 @@ our role Native[Routine $r, $libname where Str|Callable|List|IO::Path|Distributi
     has $!cpp-name-mangler;
     has Pointer $!entry-point;
     has int $!arity;
-    has int8 $!is-clone;
-    has int8 $!any-optionals;
+    has int $!is-clone;
+    has int $!any-optionals;
     has Mu $!optimized-body;
     has Mu $!jit-optimized-body;