sparc: Use set_current_blocked()

As described in e6fa16ab ("signal: sigprocmask() should do
retarget_shared_pending()") the modification of current->blocked is
incorrect as we need to check whether the signal we're about to block
is pending in the shared queue.

Cc: Oleg Nesterov <oleg@redhat.com>
Cc: "David S. Miller" <davem@davemloft.net>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>