unix: add KexecFileLoad on linux/{amd64,ppc64x,s390x}

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>

unix/syscall_linux_amd64.go

@@ -160,3 +160,16 @@ func Poll(fds []PollFd, timeout int) (n int, err error) {
 	}
 	return poll(&fds[0], len(fds), timeout)
 }
+
+//sys	kexecFileLoad(kernelFd int, initrdFd int, cmdlineLen int, cmdline string, flags int) (err error)
+
+func KexecFileLoad(kernelFd int, initrdFd int, cmdline string, flags int) error {
+	cmdlineLen := len(cmdline)
+	if cmdlineLen > 0 {
+		// Account for the additional NULL byte added by
+		// BytePtrFromString in kexecFileLoad. The kexec_file_load
+		// syscall expects a NULL-terminated string.
+		cmdlineLen++
+	}
+	return kexecFileLoad(kernelFd, initrdFd, cmdlineLen, cmdline, flags)
+}