target-s390x: Fix duplicate call of tcg_temp_new_i64

tmp2 = tcg_temp_new_i64() is already executed unconditionally,
so there is no need to call it a second time for 64 bit hosts.

Signed-off-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: Alexander Graf <agraf@suse.de>