fix(cmd): usage exit codes, asm output file and cross-arch ground truth

Assisted-by: GLM 5.3

cmd/gasm/scaffold.go

@@ -44,7 +44,7 @@ bodies, place the file in the kernel's package, and run it in CI.
 		rest = rest[1:]
 	}
 	if len(rest) != 1 {
-		return fmt.Errorf("usage: gasm scaffold differential <file.s>")
+		return &usageError{fmt.Errorf("usage: gasm scaffold differential <file.s>")}
 	}
 	path := rest[0]
 	src, err := os.ReadFile(path)