Add a wrapper for the memfd_create syscall on Linux. See
http://man7.org/linux/man-pages/man2/memfd_create.2.html for more
information.

Also add the corresponding MFD_* constants to be used in the flags
argument.

Change-Id: Ib30a007e15575a38ecf204601ffede9c913799b6
Reviewed-on: https://go-review.googlesource.com/135915
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>