This CL introduces the Getrandom syscall. Allowing users of
sufficiently new kernels (3.17) to detect when the operating system
lacks enough entropy to generate cryptographic random data. See:
http://man7.org/linux/man-pages/man2/getrandom.2.html

As we have updated the syscall numbers, the Getrandom function can
just be generated normally. The only other change is to mkerrors.sh
to include the GRND_* flags for getrandom from <linux/random.h>.

It is important to note that currently the mkerrors.sh script cannot
actually be run as it has fallen behind the 4.10 kernel. The GRND_*
flags were added manually.

This was originally part of a larger commit that was split up.
See CL: https://go-review.googlesource.com/37570

Change-Id: I464088a9c9f7224abdcdae476b30a4f2f412d9f5
Reviewed-on: https://go-review.googlesource.com/37589Reviewed-by: Matt Layher <mdlayher@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Matt Layher <mdlayher@gmail.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>