cmd/link: never coalesce type descriptors when dynamically linking Go

Add a test by making misc/cgo/testshared/src/trivial.go marginally less
trivial.

Fixes #25970.

Change-Id: I8815d0c56b8850fcdbf9b45f8406f37bd21b6865
Reviewed-on: https://go-review.googlesource.com/120235
Run-TryBot: Michael Hudson-Doyle <michael.hudson@canonical.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

misc/cgo/testshared/src/trivial/trivial.go

 package main
 
 func main() {
+	// This is enough to make sure that the executable references
+	// a type descriptor, which was the cause of
+	// https://golang.org/issue/25970.
+	c := make(chan int)
+	_ = c
 }

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

@@ -368,28 +368,30 @@ func (ctxt *Link) symtab() {
 	// pseudo-symbols to mark locations of type, string, and go string data.
 	var symtype *sym.Symbol
 	var symtyperel *sym.Symbol
-	if ctxt.UseRelro() && (ctxt.BuildMode == BuildModeCArchive || ctxt.BuildMode == BuildModeCShared || ctxt.BuildMode == BuildModePIE) {
-		s = ctxt.Syms.Lookup("type.*", 0)
+	if !ctxt.DynlinkingGo() {
+		if ctxt.UseRelro() && (ctxt.BuildMode == BuildModeCArchive || ctxt.BuildMode == BuildModeCShared || ctxt.BuildMode == BuildModePIE) {
+			s = ctxt.Syms.Lookup("type.*", 0)
 
-		s.Type = sym.STYPE
-		s.Size = 0
-		s.Attr |= sym.AttrReachable
-		symtype = s
+			s.Type = sym.STYPE
+			s.Size = 0
+			s.Attr |= sym.AttrReachable
+			symtype = s
 
-		s = ctxt.Syms.Lookup("typerel.*", 0)
+			s = ctxt.Syms.Lookup("typerel.*", 0)
 
-		s.Type = sym.STYPERELRO
-		s.Size = 0
-		s.Attr |= sym.AttrReachable
-		symtyperel = s
-	} else if !ctxt.DynlinkingGo() {
-		s = ctxt.Syms.Lookup("type.*", 0)
+			s.Type = sym.STYPERELRO
+			s.Size = 0
+			s.Attr |= sym.AttrReachable
+			symtyperel = s
+		} else {
+			s = ctxt.Syms.Lookup("type.*", 0)
 
-		s.Type = sym.STYPE
-		s.Size = 0
-		s.Attr |= sym.AttrReachable
-		symtype = s
-		symtyperel = s
+			s.Type = sym.STYPE
+			s.Size = 0
+			s.Attr |= sym.AttrReachable
+			symtype = s
+			symtyperel = s
+		}
 	}
 
 	groupSym := func(name string, t sym.SymKind) *sym.Symbol {