unix: Add UnlinkAt on Linux with third flags argument

For golang/go#9923

Change-Id: I7562d32f1bfcc35fe86d2e5fff26845737fd9ecb
Reviewed-on: https://go-review.googlesource.com/7917Reviewed-by: Rob Pike <r@golang.org>

unix/syscall_linux.go

@@ -869,7 +869,7 @@ func Setgid(uid int) (err error) {
 //sysnb	Umask(mask int) (oldmask int)
 //sysnb	Uname(buf *Utsname) (err error)
 //sys	Unlink(path string) (err error)
-//sys	Unlinkat(dirfd int, path string) (err error)
+//sys	Unlinkat(dirfd int, path string, flags int) (err error)
 //sys	Unmount(target string, flags int) (err error) = SYS_UMOUNT2
 //sys	Unshare(flags int) (err error)
 //sys	Ustat(dev int, ubuf *Ustat_t) (err error)

unix/zsyscall_linux_386.go

@@ -1190,13 +1190,13 @@ func Unlink(path string) (err error) {
 
 // THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
 
-func Unlinkat(dirfd int, path string) (err error) {
+func Unlinkat(dirfd int, path string, flags int) (err error) {
 	var _p0 *byte
 	_p0, err = BytePtrFromString(path)
 	if err != nil {
 		return
 	}
-	_, _, e1 := Syscall(SYS_UNLINKAT, uintptr(dirfd), uintptr(unsafe.Pointer(_p0)), 0)
+	_, _, e1 := Syscall(SYS_UNLINKAT, uintptr(dirfd), uintptr(unsafe.Pointer(_p0)), uintptr(flags))
 	use(unsafe.Pointer(_p0))
 	if e1 != 0 {
 		err = e1

unix/zsyscall_linux_amd64.go

@@ -1190,13 +1190,13 @@ func Unlink(path string) (err error) {
 
 // THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
 
-func Unlinkat(dirfd int, path string) (err error) {
+func Unlinkat(dirfd int, path string, flags int) (err error) {
 	var _p0 *byte
 	_p0, err = BytePtrFromString(path)
 	if err != nil {
 		return
 	}
-	_, _, e1 := Syscall(SYS_UNLINKAT, uintptr(dirfd), uintptr(unsafe.Pointer(_p0)), 0)
+	_, _, e1 := Syscall(SYS_UNLINKAT, uintptr(dirfd), uintptr(unsafe.Pointer(_p0)), uintptr(flags))
 	use(unsafe.Pointer(_p0))
 	if e1 != 0 {
 		err = e1

unix/zsyscall_linux_arm.go

@@ -1190,13 +1190,13 @@ func Unlink(path string) (err error) {
 
 // THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
 
-func Unlinkat(dirfd int, path string) (err error) {
+func Unlinkat(dirfd int, path string, flags int) (err error) {
 	var _p0 *byte
 	_p0, err = BytePtrFromString(path)
 	if err != nil {
 		return
 	}
-	_, _, e1 := Syscall(SYS_UNLINKAT, uintptr(dirfd), uintptr(unsafe.Pointer(_p0)), 0)
+	_, _, e1 := Syscall(SYS_UNLINKAT, uintptr(dirfd), uintptr(unsafe.Pointer(_p0)), uintptr(flags))
 	use(unsafe.Pointer(_p0))
 	if e1 != 0 {
 		err = e1