softmmu: support up to 12 MMU modes

At 8k per TLB (for 64-bit host or target), 8 or more modes
make the TLBs bigger than 64k, and some RISC TCG backends do
not like that.  On the affected hosts, cut the TLB size in
half---there is still a measurable speedup on PPC with the
next patch.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <1424436345-37924-3-git-send-email-pbonzini@redhat.com>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Alexander Graf <agraf@suse.de>