ia32/restorer: move 32-bit pie unmap to x86

We need compat realization for restorer unmap as after rt_sigreturn()
the task is stopped it 32-bit code and ptrace API doesn't allow
setting x86_64 full registers set to ia32 task.

Generic restorer has now x86-specific __export_unmap_compat()
function, which isn't right.

Clean restorer from x86-related realization.

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