On MIPS, pipe returns two values rather than taking a pointer.

On MIPS64, call pipe2 rather than pipe.

Also, use the correct system call number for fcntl on mips64.

Change-Id: Ie72acdefeb593f44cb98735fc40eac99cf73509e
Reviewed-on: https://go-review.googlesource.com/c/go/+/202417
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>