tcg: Add type for vCPU pointers

Adds the 'TCGv_env' type for pointers to 'CPUArchState' objects. The
tracing infrastructure later needs to differentiate between regular
pointers and pointers to vCPUs.

Also changes all targets to use the new 'TCGv_env' type instead of the
generic 'TCGv_ptr'. As of now, the change is merely cosmetic ('TCGv_env'
translates into 'TCGv_ptr'), but that could change in the future to
enforce the difference.

Note that a 'TCGv_env' type (for 'CPUState') is not added, since all
helpers currently receive the architecture-specific
pointer ('CPUArchState').

Signed-off-by: Lluís Vilanova <vilanova@ac.upc.edu>
Acked-by: Richard Henderson <rth@twiddle.net>
Message-id: 145641859552.30295.7821536833590725201.stgit@localhost
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>