ia32: Rename has_compat_sigreturn => compat_cr

This naming is left from the first compatible kernel patches.
At that time to return to 32-bit task rt_sigreturn was used with
a special flag.
Now it's not true anymore, the naming doesn't relate.

Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>