Fixes build from https://golang.org/cl/12152

Plan 9 lacks syscall.EPIPE. I was misled by api/go1.txt and also
forgot to use the trybots. :(

Change-Id: I4982fe969ad4a8724090cb03009bfb21780d8aa7
Reviewed-on: https://go-review.googlesource.com/12153
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Rob Pike <r@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>