Make the amd64 post-halt-press-a-key code match i386, and introduce
into both a message for the case where cngetc() doesn't work. If
there's no console attached, this won't accomplish anything; but if
there's a screen but no keyboard, or the keyboard's wedged, or
whatever, it might provide useful information.

Suggested back in 2009 by some stuff in PR 37924 and has been hanging
about in one of my trees ever since.

(dholland)