liblink: fix field tracking

The USEFIELD instructions no longer make it to the linker,
so we have to do something else to pin the references
they were pinning. Emit a 0-length relocation of type R_USEFIELD.

Fixes #7486.

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews, r
https://golang.org/cl/95530043

include/link.h

@@ -243,6 +243,7 @@ enum
 	R_PLT0,
 	R_PLT1,
 	R_PLT2,
+	R_USEFIELD,
 };
 
 // Auto.type

src/liblink/asm6.c

@@ -3443,6 +3443,15 @@ asmins(Link *ctxt, Prog *p)
 	ctxt->andptr = ctxt->and;
 	ctxt->asmode = p->mode;
 	
+	if(p->as == AUSEFIELD) {
+		r = addrel(ctxt->cursym);
+		r->off = 0;
+		r->siz = 0;
+		r->sym = p->from.sym;
+		r->type = R_USEFIELD;
+		return;
+	}
+	
 	if(ctxt->headtype == Hnacl) {
 		if(p->as == AREP) {
 			ctxt->rep++;

src/liblink/asm8.c

@@ -2744,7 +2744,18 @@ static uchar naclret[] = {
 static void
 asmins(Link *ctxt, Prog *p)
 {
+	Reloc *r;
+
 	ctxt->andptr = ctxt->and;
+	
+	if(p->as == AUSEFIELD) {
+		r = addrel(ctxt->cursym);
+		r->off = 0;
+		r->sym = p->from.sym;
+		r->type = R_USEFIELD;
+		r->siz = 0;
+		return;
+	}
 
 	if(ctxt->headtype == Hnacl) {
 		switch(p->as) {