syscall: implement pipe() on linux/mips

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 work on kernels <2.6.27 when pipe2() was introduced.

Change-Id: I65dfbd2a02b64e777a8eb13013d718e356521be6
GitHub-Last-Rev: c483a06168387c2cf151b6419e4e16576fab640a
GitHub-Pull-Request: golang/go#26608
Reviewed-on: https://go-review.googlesource.com/125915


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Vladimir Stefanovic <vladimir.stefanovic@mips.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>