x86: Fixed remote debugging of simulated code

x86: Fixed remote debugging of simulated code

GDB breaks if more bytes are sent than the transmitted registers
actually need. Therefore the GdbRegCache struct needs to be packed to
prevent padding at the end.

Change-Id: Ib2c14eb70becdac609eb4f475d5dddbd5bcc60da
Signed-off-by: Matthias Hille <matthiashille8@gmail.com>
Reviewed-on: https://gem5-review.googlesource.com/3020
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>