x86: Make Alt-SysRq-p display the debug register contents

This patch (as921) adds code to the show_regs() routine in i386 and x86_64
to print the contents of the debug registers along with all the others.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>