Add a wrapper for the kexec_file_load syscall on the architectures that
implement it (amd64, ppc64x, s390x). See
http://man7.org/linux/man-pages/man2/kexec_file_load.2.html

Change-Id: Ie29f871902176610a6ba7ed0a7bcc95c10eb172b
Reviewed-on: https://go-review.googlesource.com/c/140037
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>