os,syscall: fix plan 9 build

NewFile take uintptr
make syscall.ProcAttr.Files be []uintptr

R=rsc
CC=golang-dev
https://golang.org/cl/5656073
diff --git a/src/pkg/os/exec_plan9.go b/src/pkg/os/exec_plan9.go
index 08f16b8..b725aeb 100644
--- a/src/pkg/os/exec_plan9.go
+++ b/src/pkg/os/exec_plan9.go
@@ -20,18 +20,10 @@
 		Sys: attr.Sys,
 	}
 
-	// Create array of integer (system) fds.
-	intfd := make([]int, len(attr.Files))
-	for i, f := range attr.Files {
-		if f == nil {
-			intfd[i] = -1
-		} else {
-			intfd[i] = f.Fd()
-		}
+	for _, f := range attr.Files {
+		sysattr.Files = append(sysattr.Files, f.Fd())
 	}
 
-	sysattr.Files = intfd
-
 	pid, h, e := syscall.StartProcess(name, argv, sysattr)
 	if e != nil {
 		return nil, &PathError{"fork/exec", name, e}