gdbstub: Change gdbserver_fork() to accept cpu instead of env

gdbstub: Change gdbserver_fork() to accept cpu instead of env

All callsites to this function navigate the cpu->env_ptr only for the
function to take the env ptr back to the original cpu ptr. Change the
function to just pass in the CPU pointer instead. Removes a core code
usage of ENV_GET_CPU() (in gdbstub.c).

Cc: Riku Voipio <riku.voipio@iki.fi>
Reviewed-by: Andreas Färber <afaerber@suse.de>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Peter Crosthwaite <crosthwaite.peter@gmail.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>