target-ppc: Change default cpu for ppc64le-linux-user

The default, 970fx, doesn't support MSR_LE.  So even though we set LE in
ppc_cpu_reset, it gets cleared again in hreg_store_msr.  Error out if a
user-selected cpu model doesn't support LE.

Signed-off-by: Richard Henderson <rth@twiddle.net>
[agraf: switch to POWER7 as default for BE and LE]
Signed-off-by: Alexander Graf <agraf@suse.de>