x86: Merge show_regs()

Using kernel_stack_pointer() allows 32-bit and 64-bit versions to
be merged.  This is more correct for 64-bit, since the old %rsp is
always saved on the stack.

Signed-off-by: Brian Gerst <brgerst@gmail.com>
LKML-Reference: <1263397555-27695-1-git-send-email-brgerst@gmail.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>