x86: get rid of put_user_try in __setup_rt_frame() (both 32bit and 64bit)

Straightforward, except for save_altstack_ex() stuck in those.
Replace that thing with an analogue that would use unsafe_put_user()
instead of put_user_ex() (called compat_save_altstack()) and be done
with that.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>