[dev.power64] liblink: fix Solaris build

a->class is a char.  Boo hoo.

LGTM=minux
R=rsc, minux
CC=golang-codereviews
https://golang.org/cl/169630043
diff --git a/src/liblink/list6.c b/src/liblink/list6.c
index d6b91c7..54cce4a 100644
--- a/src/liblink/list6.c
+++ b/src/liblink/list6.c
@@ -156,7 +156,7 @@
 			s += sprint(s, "%d ", i);
 		s += sprint(s, "offset=%ld etype=%E width=%d", a->offset, a->etype, a->width);
 		if(a->class != 0)
-			s += sprint(s, " class=%s", cnames9[a->class]);
+			s += sprint(s, " class=%s", cnames9[(int)a->class]);
 		if(a->sym != nil)
 			s += sprint(s, " sym=%s", a->sym->name);
 		if(a->type == D_BRANCH && a->u.branch != nil)