cgo: allow .so in subdirectories

R=r
https://golang.org/cl/157103

src/Make.pkg

@@ -88,7 +88,7 @@ dir:
 #	x.cgo4.c - C implementations compiled with gcc to create dynamic library
 #
 %.cgo1.go %.cgo2.go %.cgo3.c %.cgo4.c: %.go
-	cgo $(CGO_CFLAGS) $*.go
+	CGOPKGPATH=$(dir) cgo $(CGO_CFLAGS) $*.go
 
 # The rules above added x.cgo1.go and x.cgo2.go to $(GOFILES),
 # added x.cgo3.$O to $OFILES, and added the installed copy of

src/cmd/cgo/main.go

@@ -101,6 +101,6 @@ func main() {
 		os.Exit(2)
 	}
 
-	p.PackagePath = p.Package;
+	p.PackagePath = os.Getenv("CGOPKGPATH") + "/" + p.Package;
 	p.writeOutput(input);
 }