liblink: require DATA lines to be ordered by offset, with no overlap

The assembler could give a better error, but this one
is good enough for now.

Fixes #8880.

LGTM=r
R=r
CC=golang-codereviews
https://golang.org/cl/153610043

include/link.h

@@ -373,6 +373,7 @@ struct	Link
 	char*	trimpath;
 	char*	goroot;
 	char*	goroot_final;
+	int32	enforce_data_order;	// for use by assembler
 
 	// hash table of all symbols
 	LSym*	hash[LINKHASH];
@@ -542,6 +543,7 @@ vlong	adduint8(Link *ctxt, LSym *s, uint8 v);
 vlong	adduintxx(Link *ctxt, LSym *s, uint64 v, int wid);
 void	mangle(char *file);
 void	savedata(Link *ctxt, LSym *s, Prog *p, char *pn);
+void	savedata1(Link *ctxt, LSym *s, Prog *p, char *pn, int enforce_order);
 vlong	setaddr(Link *ctxt, LSym *s, vlong off, LSym *t);
 vlong	setaddrplus(Link *ctxt, LSym *s, vlong off, LSym *t, vlong add);
 vlong	setuint16(Link *ctxt, LSym *s, vlong r, uint16 v);

src/cmd/5a/lex.c

@@ -85,6 +85,7 @@ main(int argc, char *argv[])
 	ctxt = linknew(&linkarm);
 	ctxt->diag = yyerror;
 	ctxt->bso = &bstdout;
+	ctxt->enforce_data_order = 1;
 	Binit(&bstdout, 1, OWRITE);
 	listinit5();
 	fmtinstall('L', Lconv);

src/cmd/6a/lex.c

@@ -101,6 +101,7 @@ main(int argc, char *argv[])
 	ctxt = linknew(thelinkarch);
 	ctxt->diag = yyerror;
 	ctxt->bso = &bstdout;
+	ctxt->enforce_data_order = 1;
 	Binit(&bstdout, 1, OWRITE);
 	listinit6();
 	fmtinstall('L', Lconv);

src/cmd/8a/lex.c

@@ -90,6 +90,7 @@ main(int argc, char *argv[])
 	ctxt = linknew(&link386);
 	ctxt->diag = yyerror;
 	ctxt->bso = &bstdout;
+	ctxt->enforce_data_order = 1;
 	Binit(&bstdout, 1, OWRITE);
 	listinit8();
 	fmtinstall('L', Lconv);

src/liblink/data.c

@@ -83,6 +83,8 @@ savedata(Link *ctxt, LSym *s, Prog *p, char *pn)
 	siz = ctxt->arch->datasize(p);
 	if(off < 0 || siz < 0 || off >= 1<<30 || siz >= 100)
 		mangle(pn);
+	if(ctxt->enforce_data_order && off < s->np)
+		ctxt->diag("data out of order (already have %d)\n%P", p);
 	symgrow(ctxt, s, off+siz);
 
 	if(p->to.type == ctxt->arch->D_FCONST) {