tcg/ppc: Fix failure in tcg_out_mem_long

With rt != r0 on loads, we use rt for scratch.  If we need an index
register different from base, we can't use rt, but r0 is usable.

Signed-off-by: Richard Henderson <rth@twiddle.net>
Message-id: 1403843160-30332-1-git-send-email-rth@twiddle.net
Tested-by: Cédric Le Goater <clg@fr.ibm.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>