cpu: Make first_cpu and next_cpu CPUState

gdbstub needs to use CPUState::env_ptr for now.

Signed-off-by: Andreas Färber <afaerber@suse.de>
linux-user: Change thread_env to CPUState

Signed-off-by: Andreas Färber <afaerber@suse.de>
bsd-user: Change thread_env to CPUState

Signed-off-by: Andreas Färber <afaerber@suse.de>