target-cris: Use new qemu_ld/st opcodes

Using the new opcodes we can reduce if/else trees to
a single statement.

Reviewed-by: Edgar E. Iglesias <edgar.iglesias@gmail.com>
Cc: Edgar E. Iglesias <edgar.iglesias@gmail.com>
Signed-off-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Edgar E. Iglesias <edgar.iglesias@gmail.com>