liblink: use explicit field for globl duplicate detection

Overloading size leads to problems if clients
try to set up an LSym by hand.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/44140043

include/link.h

@@ -131,6 +131,7 @@ struct	LSym
 	uchar	hide;
 	uchar	leaf;	// arm only
 	uchar	fnptr;	// arm only
+	uchar	seenglobl;
 	int16	symid;	// for writing .5/.6/.8 files
 	int32	dynid;
 	int32	sig;

src/liblink/objfile.c

@@ -167,7 +167,8 @@ linkwriteobj(Link *ctxt, Biobuf *b)
 
 			if(p->as == ctxt->arch->AGLOBL) {
 				s = p->from.sym;
-				if(s->size) print("duplicate %P\n", p);
+				if(s->seenglobl++)
+					print("duplicate %P\n", p);
 				if(data == nil)
 					data = s;
 				else