alpha: belated ERESTART_RESTARTBLOCK race fix

same thing as had been done on other targets back in 2003 -
move setting ->restart_block.fn into {rt_,}sigreturn().

Tested-by: Michael Cree <mcree@orcon.net.nz>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Matt Turner <mattst88@gmail.com>