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/780_fmt_mods_run.c

@@ -115,7 +115,7 @@ static const struct row rows[] = {
 	  "    return 50;\n"
 	  "};\n",
 	  50,
-	  STAGE_CS, 0 },
+	  STAGE_CS | STAGE_WW, 1 },
 	{ "precision",
 	  "package main;\n"
 	  "import os;\n"
@@ -143,7 +143,7 @@ static const struct row rows[] = {
 	  "    return 51;\n"
 	  "};\n",
 	  51,
-	  STAGE_CS, 0 },
+	  STAGE_CS | STAGE_WW, 1 },
 	{ "base_hex",
 	  "package main;\n"
 	  "import os;\n"
@@ -171,7 +171,7 @@ static const struct row rows[] = {
 	  "    return 52;\n"
 	  "};\n",
 	  52,
-	  STAGE_CS, 0 },
+	  STAGE_CS | STAGE_WW, 1 },
 	{ "sign_plus",
 	  "package main;\n"
 	  "import os;\n"
@@ -199,7 +199,7 @@ static const struct row rows[] = {
 	  "    return 53;\n"
 	  "};\n",
 	  53,
-	  STAGE_CS, 0 },
+	  STAGE_CS | STAGE_WW, 1 },
 	{ "zero_pad",
 	  "package main;\n"
 	  "import os;\n"
@@ -227,7 +227,7 @@ static const struct row rows[] = {
 	  "    return 54;\n"
 	  "};\n",
 	  54,
-	  STAGE_CS, 0 },
+	  STAGE_CS | STAGE_WW, 1 },
 };
 
 static int