liblink: check for symgrow size too large

Many calls to symgrow pass a vlong value.  Change the function
to not implicitly truncate, and to instead give an error if
the value is too large.

R=golang-codereviews, gobot, rsc
CC=golang-codereviews
https://golang.org/cl/54010043

include/link.h

@@ -500,7 +500,7 @@ vlong	setuint32(Link *ctxt, LSym *s, vlong r, uint32 v);
 vlong	setuint64(Link *ctxt, LSym *s, vlong r, uint64 v);
 vlong	setuint8(Link *ctxt, LSym *s, vlong r, uint8 v);
 vlong	setuintxx(Link *ctxt, LSym *s, vlong off, uint64 v, vlong wid);
-void	symgrow(Link *ctxt, LSym *s, int32 siz);
+void	symgrow(Link *ctxt, LSym *s, vlong siz);
 
 // go.c
 void	double2ieee(uint64 *ieee, double native);

src/liblink/data.c

@@ -41,10 +41,16 @@ mangle(char *file)
 }
 
 void
-symgrow(Link *ctxt, LSym *s, int32 siz)
+symgrow(Link *ctxt, LSym *s, vlong lsiz)
 {
+	int32 siz;
+
 	USED(ctxt);
 
+	siz = (int32)lsiz;
+	if((vlong)siz != lsiz)
+		sysfatal("symgrow size %lld too long", lsiz);
+
 	if(s->np >= siz)
 		return;