[dev.power64] liblink: fix printing of branch targets

Print PC stored in target Prog* of branch instructions when
available instead of the offset stored in the branch
instruction.  The offset tends to be wrong after code
transformations, so previously this led to confusing listings.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/168980043

src/liblink/list9.c

@@ -259,11 +259,12 @@ Dconv(Fmt *fp)
 				sprint(str, "%s+%.5lux(BRANCH)", a->sym->name, v);
 			else
 				sprint(str, "%.5lux(BRANCH)", v);
-		} else
-			if(a->sym != nil)
-				sprint(str, "%s+%lld(APC)", a->sym->name, a->offset);
-			else
-				sprint(str, "%lld(APC)", a->offset);
+		} else if(a->u.branch != nil)
+			sprint(str, "%lld", a->u.branch->pc);
+		else if(a->sym != nil)
+			sprint(str, "%s+%lld(APC)", a->sym->name, a->offset);
+		else
+			sprint(str, "%lld(APC)", a->offset);
 		break;
 
 	case D_FCONST: