rt_sigaction.h: define HAVE_SA_RESTORER for most platforms

On some platforms LTP fails to compile, because (struct sigaction)
doesn't have .sa_restorer member. For example - ia64.

To overcome such failures we employ HAVE_SA_RESTORER define.

Signed-off-by: Stanislav Kholmanskikh <stanislav.kholmanskikh@oracle.com>
Tested-by: Cyril Hrubis <chrubis@suse.cz>