cmd/internal/obj: cache dwarfSym

Follow-up to review feedback from
mdempsky on CL 40507.

Reduces mutex contention by about 1%.

Change-Id: I540ea6772925f4a59e58f55a3458eff15880c328
Reviewed-on: https://go-review.googlesource.com/40575
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/internal/obj/link.go

@@ -329,6 +329,7 @@ type FuncInfo struct {
 	Text     *Prog
 	Autom    []*Auto
 	Pcln     Pcln
+	dwarfSym *LSym
 }
 
 // Attribute is a set of symbol attributes.

src/cmd/internal/obj/objfile.go

@@ -560,7 +560,10 @@ func (ctxt *Link) dwarfSym(s *LSym) *LSym {
 	if s.Type != STEXT {
 		ctxt.Diag("dwarfSym of non-TEXT %v", s)
 	}
-	return ctxt.Lookup(dwarf.InfoPrefix+s.Name, int(s.Version))
+	if s.FuncInfo.dwarfSym == nil {
+		s.FuncInfo.dwarfSym = ctxt.Lookup(dwarf.InfoPrefix+s.Name, int(s.Version))
+	}
+	return s.FuncInfo.dwarfSym
 }
 
 // populateDWARF fills in the DWARF Debugging Information Entry for TEXT symbol s.