wwstage: name vararg-gather slots via mklabel; graduate fmt 777/780/781 (#227)

wwstage named variadic-gather slots with mkvarargname off a separate
varargseq counter, never bumping the shared labelseq that names match
labels. cstage names them via mklabel (cmd/w6c/cgen.c:5427,5431), which
advances labelseq twice per gather. So by the time main.main reached its
`match (wr)`, wwstage's match-label counter ran two behind cstage's
(_4/_5/_6 vs _6/_7/_8) — a pure label-numbering divergence that kept fmt
cs/ww byte-id failing.

Drop varargseq and the mkvarargname helper; call the existing mklabel
for the two gather slots, matching cstage's order (vararg_d only when
nvar>0, vararg_sl always). The slot names are locals-table keys only —
they resolve to BP offsets and never reach the asm — so only the
labelseq advance is observable, which is exactly what realigns the
downstream match labels. cstage untouched (align wwstage up).

This was the match-label half of fmt's divergence; with the earlier
compound-assign fix it completes fmt byte-identity. Graduate
777/780/781 to STAGE_CS|STAGE_WW with byte_id, and drop the now-stale
(void)asm_byte_identical guard in 777.

test/wcc/777_fmt_handle_run.c

@@ -117,7 +117,7 @@ static const struct row rows[] = {
 	  "    return 42;\n"
 	  "};\n",
 	  42,
-	  STAGE_CS, 0 },
+	  STAGE_CS | STAGE_WW, 1 },
 	{ "fprintln_file_multi",
 	  "package main;\n"
 	  "import os;\n"
@@ -145,7 +145,7 @@ static const struct row rows[] = {
 	  "    return 43;\n"
 	  "};\n",
 	  43,
-	  STAGE_CS, 0 },
+	  STAGE_CS | STAGE_WW, 1 },
 	{ "fprint_file_raw",
 	  "package main;\n"
 	  "import os;\n"
@@ -173,7 +173,7 @@ static const struct row rows[] = {
 	  "    return 44;\n"
 	  "};\n",
 	  44,
-	  STAGE_CS, 0 },
+	  STAGE_CS | STAGE_WW, 1 },
 	{ "branched_fprintf",
 	  "package main;\n"
 	  "import os;\n"
@@ -206,7 +206,7 @@ static const struct row rows[] = {
 	  "    return 45;\n"
 	  "};\n",
 	  45,
-	  STAGE_CS, 0 },
+	  STAGE_CS | STAGE_WW, 1 },
 	{ "fprintf_stream",
 	  "package main;\n"
 	  "import os;\n"
@@ -230,7 +230,7 @@ static const struct row rows[] = {
 	  "    return 46;\n"
 	  "};\n",
 	  46,
-	  STAGE_CS, 0 },
+	  STAGE_CS | STAGE_WW, 1 },
 };
 
 static int
@@ -357,7 +357,6 @@ main(void)
 	int total = 0, fail = 0;
 	int wwpresent = (access(wdrv, X_OK) == 0);
 	int seq = 0;
-	(void)asm_byte_identical;	/* cstage-only until #226/#227 land */
 
 	for (int i = 0; i < n; i++) {
 		if (rows[i].stage_mask & STAGE_CS) {