[antlir2][toolchain] fix DEFAULT mapping

Summary:
This is never actually the configuration outside of some antlir tests that
query the target graph, which is why it didn't break anything loudly on
D67605234 which broke this.

Test Plan:
```
❯ buck audit providers fbcode//antlir/distro/toolchain/cxx:toolchain
```

Reviewed By: epilatow

Differential Revision: D67776288

fbshipit-source-id: 95c1313d9cc1d176ec291698b9df99cf0f6a15f1

antlir/distro/toolchain/cxx/defs.bzl

@@ -122,7 +122,7 @@ def image_cxx_toolchain(
             # the default when looking up this target directly (instead of
             # preconfigured as a dependency of something using an antlir
             # distro platform)
-            {"DEFAULT": ":{}--{}".format(name, oses[0].name)},
+            {"DEFAULT": ":{}--{}-{}".format(name, oses[0].name, oses[0].architectures[0].name)},
         ),
         visibility = visibility,
     )