microblaze: fix __user annotations

The microblaze signal handling code gets some __user
pointers wrong, as shown by sparse.
This adds the annotations where appropriate and
change sys_rt_sigreturn to correctly pass a user
stack down to do_sigaltstack instead of a kernel
structure.

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