Add the Msync function on Linux. See
http://man7.org/linux/man-pages/man2/msync.2.html

Updates golang/go#8245

Change-Id: I964259bafc8d4fbca09961443f21c4278bf56706
Reviewed-on: https://go-review.googlesource.com/50891Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>