cmd/go: allow to use syso files with cgo

I have C functions implemented in .syso file (rather than .so or inlined in .go file).
W/o this change the gcc invocation fails with undefined symbols.

R=minux.ma, rsc
CC=golang-dev
https://golang.org/cl/6352076

src/cmd/go/build.go

@@ -1576,6 +1576,7 @@ func (b *builder) cgo(p *Package, cgoExe, obj string, gccfiles []string) (outGo,
 		linkobj = append(linkobj, ofile)
 		outObj = append(outObj, ofile)
 	}
+	linkobj = append(linkobj, p.SysoFiles...)
 	dynobj := obj + "_cgo_.o"
 	if goarch == "arm" && goos == "linux" { // we need to use -pie for Linux/ARM to get accurate imported sym
 		cgoLDFLAGS = append(cgoLDFLAGS, "-pie")