Fixed extension memory layout on 32-bit targets.

We were failing to propagate the DefPool's platform to the MiniDescriptor builder.  This caused upb's code generators to incorrectly generate a field rep of `kUpb_FieldRep_8Byte` for pointer-typed extension fields instead of the 32-bit clean output:

```
UPB_SIZE(kUpb_FieldRep_4Byte, kUpb_FieldRep_8Byte)
```

PiperOrigin-RevId: 653263168

upb/reflection/field_def.c

@@ -982,9 +982,9 @@ void _upb_FieldDef_BuildMiniTableExtension(upb_DefBuilder* ctx,
       const upb_MiniTableEnum* subenum = _upb_EnumDef_MiniTable(f->sub.enumdef);
       sub = upb_MiniTableSub_FromEnum(subenum);
     }
-    bool ok2 = upb_MiniTableExtension_Init(desc.data, desc.size, mut_ext,
-                                           upb_MessageDef_MiniTable(f->msgdef),
-                                           sub, ctx->status);
+    bool ok2 = _upb_MiniTableExtension_Init(desc.data, desc.size, mut_ext,
+                                            upb_MessageDef_MiniTable(f->msgdef),
+                                            sub, ctx->platform, ctx->status);
     if (!ok2) _upb_DefBuilder_Errf(ctx, "Could not build extension mini table");
   }