monitor: Split mon_get_cpu fn to remove ENV_GET_CPU

The monitor currently has one helper, mon_get_cpu() which will return
an env pointer. The target specific users of this API want an env, but
all the target agnostic users really just want the cpu pointer. These
users then need to use the target-specifically defined ENV_GET_CPU to
navigate back up to the CPU from the ENV. Split the API for the two
uses cases to remove all need for ENV_GET_CPU.

Reviewed-by: Richard Henderson <rth@twiddle.net>
Reviewed-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Peter Crosthwaite <crosthwaite.peter@gmail.com>
Acked-by: Luiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>