linux-user: Fix exit syscall with QOM CPU

For QOM'ified CPUs we cannot g_free() CPUArchState, we must
object_delete() the object it is embedded into.

Fixes LP#982321 (invalid free() while executing pacman with qemu-arm).

Reported-by: Serge Schneider <serge@xecdesign.com>
Reported-by: Russell Keith Davis <russell@russelldavis.org>
Signed-off-by: Andreas Färber <afaerber@suse.de>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Tested-by: Serge Schneider <serge@xecdesign.com>
Tested-by: Russell Keith Davis <russell@russelldavis.org>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
