target/ppc: improve performance of large BAT invalidations

Performing a complete flush is ~ 100 times faster than flushing
256MiB of 4KiB pages. Set a limit of 1024 pages and perform a complete
flush afterwards.

This patch significantly speeds up AIX 5.1 and NetBSD-ofppc.

Signed-off-by: Artyom Tarasenko <atar4qemu@gmail.com>
Message-Id: <1555103178-21894-4-git-send-email-atar4qemu@gmail.com>
Reviewed-by: Hervé Poussineau <hpoussin@reactos.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>