tcg/ppc: Do not overwrite lower address word on Darwin and AIX

For targets where TARGET_LONG_BITS != 32, i.e. 64-bit guests,
addr_reg is moved to r4. For hosts without TCG_TARGET_CALL_ALIGN_ARGS
either data_reg2 or data_reg or a masked version thereof would overwrite
r4. Place it in r5 instead, matching TCG_TARGET_CALL_ALIGN_ARGS hosts.

This fixes immediate crashes of 64-bit guests observed on Darwin/ppc but
not on Darwin/ppc64.

Signed-off-by: Andreas Färber <andreas.faerber@web.de>
Acked-by: malc <av1474@comtv.ru>
Acked-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
