linux-user: target_argv is placed on ts->bprm->argv and can't be freed()

TaskState contains linux_bprm struct which encapsulates argv among
other things.
argv might be used around the code and is expected to contain valid
data. Before this patch, ts->bprm->argv was NULL due to it being
freed right after loader_exec().

Signed-off-by: Fabio Erculiani <lxnay@sabayon.org>
Acked-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
