os: move epipecheck from file_posix.go and into file_unix.go to fix windows build

R=golang-dev
CC=golang-dev
https://golang.org/cl/6450058

src/pkg/os/file_unix.go

@@ -8,6 +8,7 @@ package os
 
 import (
 	"runtime"
+	"sync/atomic"
 	"syscall"
 )
 
@@ -53,6 +54,16 @@ type dirInfo struct {
 	bufp int    // location of next record in buf.
 }
 
+func epipecheck(file *File, e error) {
+	if e == syscall.EPIPE {
+		if atomic.AddInt32(&file.nepipe, 1) >= 10 {
+			sigpipe()
+		}
+	} else {
+		atomic.StoreInt32(&file.nepipe, 0)
+	}
+}
+
 // DevNull is the name of the operating system's ``null device.''
 // On Unix-like systems, it is "/dev/null"; on Windows, "NUL".
 const DevNull = "/dev/null"