The pipe2() syscall is available on all Linux platforms since kernel
2.6.27, so use it unconditionally to emulate pipe() and pipe2().

Signed-off-by: Helge Deller <deller@gmx.de>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <YtbZ2ojisTnzxN9Y@p100>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>