microblaze: prepare signal handling for generic unistd.h

We need to define set_restore_sigmask() in order to
get pselect and ppoll. Also, the setup_frame function
can not be used when __NR_sigreturn is not defined.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Michal Simek <monstr@monstr.eu>