selfhost: port switch — N_SWITCH parser + cgen + scratch slot

test/wcc/994_w6c_ww.c

@@ -149,6 +149,16 @@ main(void)
 		  "  let m: mode = mode.RW;\n"
 		  "  return m as i32;\n"
 		  "};" },
+		{ "switch",
+		  "fn classify(x: i32) i32 = {\n"
+		  "  switch (x) {\n"
+		  "    case 1, 2, 3: return 10;\n"
+		  "    case 7: return 70;\n"
+		  "    case: return 0;\n"
+		  "  };\n"
+		  "  return -1;\n"
+		  "};\n"
+		  "fn main() i32 = { return classify(2); };" },
 		{ NULL, NULL },
 	};