syscall: fix returned values of Dup2 on aix/ppc64

Change-Id: Ia78ea589cc6d58ff22f7d519399c06c5308419dd
Reviewed-on: https://go-review.googlesource.com/c/go/+/168878Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>

src/syscall/syscall_aix.go

@@ -46,7 +46,7 @@ func (ts *StTimespec_t) Nano() int64 {
 // But, as fcntl is currently not exported and isn't called with F_DUP2FD,
 // it doesn't matter.
 //sys	fcntl(fd int, cmd int, arg int) (val int, err error)
-//sys	Dup2(old int, new int) (val int, err error)
+//sys	Dup2(old int, new int) (err error)
 
 //sysnb pipe(p *[2]_C_int) (err error)
 func Pipe(p []int) (err error) {

src/syscall/zsyscall_aix_ppc64.go

@@ -294,9 +294,8 @@ func fcntl(fd int, cmd int, arg int) (val int, err error) {
 
 // THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
 
-func Dup2(old int, new int) (val int, err error) {
-	r0, _, e1 := syscall6(uintptr(unsafe.Pointer(&libc_Dup2)), 2, uintptr(old), uintptr(new), 0, 0, 0, 0)
-	val = int(r0)
+func Dup2(old int, new int) (err error) {
+	_, _, e1 := syscall6(uintptr(unsafe.Pointer(&libc_Dup2)), 2, uintptr(old), uintptr(new), 0, 0, 0, 0)
 	if e1 != 0 {
 		err = errnoErr(e1)
 	}