target-ppc: Fix build for PPC_DEBUG_DISAS

In r5949 / 76db3ba4 (target-ppc: memory
load/store rework) variable little_endian was replaced with ctx.le_mode.
Update the debug code.

Signed-off-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Alexander Graf <agraf@suse.de>