cmd/link: fix some unintentional symbol creation

There are two places in DWARF generation that create symbols when they
really just want to get the symbol if it exists. writeranges, in
particular, will create a DWARF range symbol for every single textp
symbol (though they won't get linked into any list, so they don't
affect the binary).

Fix these to use ROLookup instead of Lookup.

Change-Id: I401eadf22890e296bd08bccaa6ba2fd8fac800cd
Reviewed-on: https://go-review.googlesource.com/69971
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>

src/cmd/link/internal/ld/dwarf.go

@@ -1273,8 +1273,8 @@ func writeframes(ctxt *Link, syms []*sym.Symbol) []*sym.Symbol {
 func writeranges(ctxt *Link, syms []*sym.Symbol) []*sym.Symbol {
 	empty := true
 	for _, s := range ctxt.Textp {
-		rangeSym := ctxt.Syms.Lookup(dwarf.RangePrefix+s.Name, int(s.Version))
-		if rangeSym.Size == 0 {
+		rangeSym := ctxt.Syms.ROLookup(dwarf.RangePrefix+s.Name, int(s.Version))
+		if rangeSym == nil || rangeSym.Size == 0 {
 			continue
 		}
 		rangeSym.Attr |= sym.AttrReachable | sym.AttrNotInSymbolTable
@@ -1555,7 +1555,7 @@ func dwarfgeneratedebugsyms(ctxt *Link) {
 
 	var consts []*sym.Symbol
 	for _, lib := range ctxt.Library {
-		if s := ctxt.Syms.Lookup(dwarf.ConstInfoPrefix+lib.Pkg, 0); s != nil {
+		if s := ctxt.Syms.ROLookup(dwarf.ConstInfoPrefix+lib.Pkg, 0); s != nil {
 			importInfoSymbol(ctxt, s)
 			consts = append(consts, s)
 		}