Restore registers not present in the sigreturn signal frame

Add new function arch_set_task_regs_nosigrt(). It allows to restore
architecture-specific registers not present in sigreturn signal frame.

Each architecture can overwrite this function.

The arch_set_task_regs_nosigrt function restores the registers between the
final sigreturn and PTRACE_DETACH. We do this at this point because
we are sure that all threads have already been created.

Add arch_set_thread_regs in pre-dump because some registers may have
been changed by the parasite infection and they are not present in
sigreturn signal frame.

Signed-off-by: Alice Frosi <alice@linux.vnet.ibm.com>
Reviewed-by: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>