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/11652
Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/internal/obj/link.go

@@ -365,9 +365,10 @@ const (
 	SCONST
 	SDYNIMPORT
 	SHOSTOBJ
-	SSUB    = 1 << 8
-	SMASK   = SSUB - 1
-	SHIDDEN = 1 << 9
+	SSUB       = 1 << 8
+	SMASK      = SSUB - 1
+	SHIDDEN    = 1 << 9
+	SCONTAINER = 1 << 10 // has a sub-symbol
 )
 
 type Reloc struct {