[release-branch.go1.14] runtime: negate errno value for mips pipe/pipe2

The callers expect negative errno values, so negate them when necessary.

No test because there is no reasonable way to make pipe/pipe2 fail.
This was reported on a system on which pipe2 returned ENOSYS.

For #37997
Fixes #38005

Change-Id: I3ad6cbbc2521cf495f8df6ec991a3f781122b508
Reviewed-on: https://go-review.googlesource.com/c/go/+/224592


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
(cherry picked from commit 20b46c7c)
Reviewed-on: https://go-review.googlesource.com/c/go/+/225419