fw_stub.awk: use @generated tag in generated files

Multiple tools use @generated to identify generated files (for example,
in a review Phabricator will by default hide diffs in enerated files).
Use the @generated tag in makesyscalls.sh as we've done for other
generated files.

sys/tools/fw_stub.awk

@@ -123,8 +123,10 @@ ctmpfilename = cfilename ".tmp";
 modname = opt_m;
 gsub(/[-\.]/, "_", modname);
 
+# Avoid a literal generated file tag here.
+generated = "@" "generated";
 printc("/*\
- * Automatically generated by:\
+ * Automatically " generated " by:\
  * $FreeBSD$\
  */");
 printc("#include <sys/param.h>");