target-ppc: change DCR helpers to target_long arguments

The recent transition to always have the DCR helper functions take 32 bit
values broke the PPC64 target, as target_long became 64 bits there.

This patch changes DCR helpers to target_long arguments, and cast the values
to 32 bit when needed.

Fixes PPC64 build with --enable-debug-tcg

Based on a patch from Alexander Graf <agraf@suse.de>
Reported-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>