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_ppc64x.go

@@ -7,6 +7,7 @@
 
 package unix
 
+//sys	Dup2(oldfd int, newfd int) (err error)
 //sys	Fchown(fd int, uid int, gid int) (err error)
 //sys	Fstat(fd int, stat *Stat_t) (err error)
 //sys	Fstatfs(fd int, buf *Statfs_t) (err error)