cmd/link: correct runtime/cgo detection for -linkshared

Current code detect runtime/cgo iff the package or sub packages imports
runtime/cgo directly. However, when we are using linkshared, imported
shared libraries might have already included runtime/cgo.
This CL handles later case by looking an actual runtime/cgo symbol.

Change-Id: I35e7dfdb5e1a939eafc95a0259ee1af9782bc864
Reviewed-on: https://go-review.googlesource.com/56310Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -421,7 +421,6 @@ func (ctxt *Link) loadlib() {
 
 	var i int
 	for i = 0; i < len(ctxt.Library); i++ {
-		iscgo = iscgo || ctxt.Library[i].Pkg == "runtime/cgo"
 		if ctxt.Library[i].Shlib == "" {
 			if ctxt.Debugvlog > 1 {
 				ctxt.Logf("%5.2f autolib: %s (from %s)\n", Cputime(), ctxt.Library[i].File, ctxt.Library[i].Objref)
@@ -439,6 +438,8 @@ func (ctxt *Link) loadlib() {
 		}
 	}
 
+	iscgo = ctxt.Syms.ROLookup("x_cgo_init", 0) != nil
+
 	// We now have enough information to determine the link mode.
 	determineLinkMode(ctxt)