cmd/compile: only generate ·f symbols when necessary

Before go supported buildmode=shared ·f symbols used to be defined
only when they were used. In order to solve #11480 the strategy
was changed to have these symbols defined on declaration which is
less efficient and generates many unneeded symbols.
With this change the best strategy is chosen for each situation,
improving static linking time:

name            old s/op    new s/op    delta
LinkCmdCompile   0.27 ± 5%   0.25 ± 6%  -8.22%   (p=0.000 n=98+96)
LinkCmdGo        0.30 ± 6%   0.29 ± 8%  -5.03%   (p=0.000 n=95+99)

name            old MaxRSS  new MaxRSS  delta
LinkCmdCompile   107k ± 2%    98k ± 3%  -8.32%  (p=0.000 n=99+100)
LinkCmdGo        106k ± 3%   104k ± 3%  -1.94%  (p=0.000 n=99+100)

Change-Id: I965eeee30541e724fd363804adcd6fda10f965a4
Reviewed-on: https://go-review.googlesource.com/31031Reviewed-by: Michael Hudson-Doyle <michael.hudson@canonical.com>

src/cmd/compile/internal/gc/closure.go

@@ -218,7 +218,9 @@ func makeclosure(func_ *Node) *Node {
 	xfunc.Func.Nname.Name.Funcdepth = func_.Func.Depth
 	xfunc.Func.Depth = func_.Func.Depth
 	xfunc.Func.Endlineno = func_.Func.Endlineno
-	makefuncsym(xfunc.Func.Nname.Sym)
+	if Ctxt.Flag_dynlink {
+		makefuncsym(xfunc.Func.Nname.Sym)
+	}
 
 	xfunc.Nbody.Set(func_.Nbody.Slice())
 	xfunc.Func.Dcl = append(func_.Func.Dcl, xfunc.Func.Dcl...)

src/cmd/compile/internal/gc/dcl.go

@@ -526,7 +526,7 @@ func funchdr(n *Node) {
 		Fatalf("funchdr: dclcontext = %d", dclcontext)
 	}
 
-	if importpkg == nil && n.Func.Nname != nil {
+	if Ctxt.Flag_dynlink && importpkg == nil && n.Func.Nname != nil {
 		makefuncsym(n.Func.Nname.Sym)
 	}
 
@@ -1318,6 +1318,11 @@ func funcsym(s *Sym) *Sym {
 	}
 
 	s1 := Pkglookup(s.Name+"·f", s.Pkg)
+	if !Ctxt.Flag_dynlink && s1.Def == nil {
+		s1.Def = newfuncname(s1)
+		s1.Def.Func.Shortname = newname(s)
+		funcsyms = append(funcsyms, s1.Def)
+	}
 	s.Fsym = s1
 	return s1
 }