x86/restorer/compat: let stack32 be per-thread

As threads restore in parallel, stack32 may be reused concurrently
leading to reusing others thread's data. So, let it lay on stack.
It would still worth making 32-bit stack per-task reusing it in threads
but at this moment introducing such complexity looks like premature
optimization.
It does not affect 64-bit C/R.

Fixes: file_aio, sigaltstack, clone_fs, socket_aio, different_creds, futex

travis-ci: success for 32-bit tests fixes
Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>