Change the Pipe() function to use the pipe() syscall (which has a unique
calling convention on linux/mips) instead of using pipe2(). This allows
it to work on kernels <2.6.27 when pipe2() was introduced.

Change-Id: I088886e66b594232869d8b88c6e1a630c76188a3
Reviewed-on: https://go-review.googlesource.com/126176Reviewed-by: Vladimir Stefanovic <vladimir.stefanovic@mips.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>