x/sys/unix: add Dup2 wrapper for arm64/ppc64

arm64 doesn't have a Dup2 syscall, instead Dup3 is supposed
to be used. Since Dup3 is linux-specific, provide a wrapper
to make writing portable code easier.

    Updates golang/go#10235

To verify it, added a testcase for Dup and Dup2.

Change-Id: I066bb60d62b2bd64d7ba0fdfbb334ce2213c78e9
Reviewed-on: https://go-review.googlesource.com/20178
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Riku Voipio <riku.voipio@linaro.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

unix/syscall_linux_arm64.go

@@ -152,6 +152,10 @@ func InotifyInit() (fd int, err error) {
 	return InotifyInit1(0)
 }
 
+func Dup2(oldfd int, newfd int) (err error) {
+	return Dup3(oldfd, newfd, 0)
+}
+
 // TODO(dfc): constants that should be in zsysnum_linux_arm64.go, remove
 // these when the deprecated syscalls that the syscall package relies on
 // are removed.