cmd/link/internal/ld: exclude only real container symbols from symtab

It looks like the test for whether symbols contain subsymbols is wrong.
In particular, symbols in C libraries are mistakenly considered container
symbols.

Fix the test so only symbols which actually have a subsymbol
are excluded from the symtab.  When linking cgo programs the list
of containers is small, something like:

container _/home/khr/sandbox/symtab/misc/cgo/test(.text)<74>
container _/home/khr/sandbox/symtab/misc/cgo/test/issue8828(.text)<75>
container _/home/khr/sandbox/symtab/misc/cgo/test/issue9026(.text)<76>
container runtime/cgo(.text)<77>

I'm not sure this is the right fix.  In particular I can't reproduce
the original problem.  Anyone have a repro they can try and see if
this fix works?

Fixes #10747
Fixes #11396

Change-Id: Id8b016389d33348b4a791fdcba0f9db8ae71ebf3
Reviewed-on: https://go-review.googlesource.com/11652Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/internal/obj/link.go

@@ -368,6 +368,7 @@ const (
 	SSUB       = 1 << 8
 	SMASK      = SSUB - 1
 	SHIDDEN    = 1 << 9
+	SCONTAINER = 1 << 10 // has a sub-symbol
 )
 
 type Reloc struct {

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

@@ -191,7 +191,7 @@ func renumberfiles(ctxt *Link, files []*LSym, d *Pcdata) {
 func container(s *LSym) int {
 	// We want to generate func table entries only for the "lowest level" symbols,
 	// not containers of subsymbols.
-	if s != nil && s.Sub != nil {
+	if s != nil && s.Type&obj.SCONTAINER != 0 {
 		return 1
 	}
 	return 0
@@ -223,6 +223,13 @@ func pclntab() {
 	//	offset to file table [4 bytes]
 	nfunc := int32(0)
 
+	// Find container symbols, mark them with SCONTAINER
+	for Ctxt.Cursym = Ctxt.Textp; Ctxt.Cursym != nil; Ctxt.Cursym = Ctxt.Cursym.Next {
+		if Ctxt.Cursym.Outer != nil {
+			Ctxt.Cursym.Outer.Type |= obj.SCONTAINER
+		}
+	}
+
 	for Ctxt.Cursym = Ctxt.Textp; Ctxt.Cursym != nil; Ctxt.Cursym = Ctxt.Cursym.Next {
 		if container(Ctxt.Cursym) == 0 {
 			nfunc++