cmd/gc: do not mark PC as pointer on stack

GC thinks that it's an invalid pointer on plan9:
http://build.golang.org/log/f7da638a20544ef7f45e81f23fdbc4875044c296

LGTM=0intro
R=golang-codereviews, 0intro
CC=aram, golang-codereviews
https://golang.org/cl/113350043

src/cmd/gc/select.c

@@ -347,7 +347,7 @@ selecttype(int32 size)
 	scase = nod(OTSTRUCT, N, N);
 	scase->list = list(scase->list, nod(ODCLFIELD, newname(lookup("sg")), sudog));
 	scase->list = list(scase->list, nod(ODCLFIELD, newname(lookup("chan")), typenod(ptrto(types[TUINT8]))));
-	scase->list = list(scase->list, nod(ODCLFIELD, newname(lookup("pc")), typenod(ptrto(types[TUINT8]))));
+	scase->list = list(scase->list, nod(ODCLFIELD, newname(lookup("pc")), typenod(types[TUINTPTR])));
 	scase->list = list(scase->list, nod(ODCLFIELD, newname(lookup("kind")), typenod(types[TUINT16])));
 	scase->list = list(scase->list, nod(ODCLFIELD, newname(lookup("so")), typenod(types[TUINT16])));
 	scase->list = list(scase->list, nod(ODCLFIELD, newname(lookup("receivedp")), typenod(ptrto(types[TUINT8]))));