kexec, x86: Fix bzImage real-mode booting

We need to keep space for bss, heap/stack before command line.
otherwise command_line will be cleared by kernel 16bit init code.

also need to set 32bit start in real_mode header, kernel 16bit code
need to jump there.

Also don't touch regs16 if --real-mode is not specified.

Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Signed-off-by: Simon Horman <horms@verge.net.au>