fix(format): never peel a stacked label that names a macro

Peeling a stacked label whose name is a macro moves it onto a line of
its own, where its expansion decides the line's shape: an empty body
leaves a bare colon behind, a line the parser rejects.  The peel loops
now hold such labels back with the rest of the line, and the crashing
input joins the corpus.

Assisted-by: GLM 5.3

format/expansion_test.go

@@ -117,6 +117,16 @@ func TestFormatPreservesExpansionSemantics(t *testing.T) {
 				"\tA: M\n" +
 				"\tRET\n",
 		},
+		{
+			// A stacked label naming a macro: its expansion decides what
+			// the line becomes (an empty body makes it vanish), so it is
+			// never peeled onto a line of its own.
+			name: "stacked label naming a macro",
+			src: "#define M\n" +
+				"TEXT ·f(SB), $0\n" +
+				"\tA: M:\n" +
+				"\tRET\n",
+		},
 		{
 			// A use above its define: the name is not a macro at that point
 			// of the file, so splitting would be safe; the whole-file set