os,syscall: fix windows build

make syscall.ProcAttr.Files be []uintptr

all.bash passes on Linux.
things seem to compile on GOOS={darwin,windows}

R=golang-dev, mattn.jp, alex.brainman, rsc
CC=golang-dev
https://golang.org/cl/5653055

src/pkg/syscall/exec_unix.go

@@ -101,9 +101,9 @@ type Credential struct {
 // ProcAttr holds attributes that will be applied to a new process started
 // by StartProcess.
 type ProcAttr struct {
-	Dir   string   // Current working directory.
-	Env   []string // Environment.
-	Files []int    // File descriptors.
+	Dir   string    // Current working directory.
+	Env   []string  // Environment.
+	Files []uintptr // File descriptors.
 	Sys   *SysProcAttr
 }