x86-64, reboot: Be more paranoid in 64-bit reboot=bios

Be a bit more paranoid in the transition back to 16-bit mode.  In
particular, in case the kernel is residing above the 4 GiB mark,
switch to the trampoline GDT, and make the jump after turning off
paging a far jump.  In theory, none of this should matter, but it is
exactly the kind of things that broken SMM or virtualization software
could trip up on.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Link: http://lkml.kernel.org/r/tip-jopx7y6g6dbcx4tpal8q0jlr@git.kernel.org