tcg: add TCG_TARGET_TLB_DISPLACEMENT_BITS

This will be used to size the TLB when more than 8 MMU modes are
used by the target.  Limitations come from the limited size of
the immediate fields (which sometimes, as in the case of Aarch64,
extend to instructions that shift the immediate).

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <1424436345-37924-2-git-send-email-pbonzini@redhat.com>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Alexander Graf <agraf@suse.de>
