cmd/link: do not mark go.plugin.tabs as reachable in non-plugins

Fixes #18250

Change-Id: I4f61591356ddb4a906c206ad8456d1839daf7b91
Reviewed-on: https://go-review.googlesource.com/34170
Run-TryBot: Michael Hudson-Doyle <michael.hudson@canonical.com>
Reviewed-by: David Crawshaw <crawshaw@golang.org>

misc/cgo/testshared/src/exe/exe.go

@@ -7,6 +7,11 @@ import (
 	"runtime"
 )
 
+// Having a function declared in the main package triggered
+// golang.org/issue/18250
+func DeclaredInMain() {
+}
+
 func main() {
 	defer depBase.ImplementedInAsm()
 	// This code below causes various go.itab.* symbols to be generated in

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

@@ -602,8 +602,7 @@ func (ctxt *Link) symtab() {
 	adduint(ctxt, moduledata, uint64(nitablinks))
 	adduint(ctxt, moduledata, uint64(nitablinks))
 	// The ptab slice
-	if ptab := ctxt.Syms.ROLookup("go.plugin.tabs", 0); ptab != nil {
-		ptab.Attr |= AttrReachable
+	if ptab := ctxt.Syms.ROLookup("go.plugin.tabs", 0); ptab != nil && ptab.Attr.Reachable() {
 		ptab.Attr |= AttrLocal
 		ptab.Type = obj.SRODATA