[alpha] missing semtimedop

[Roland McGrath]

> Should I bother with the typing, or just add the one syscall?

Might as well have the typing, since other arch's do.  It's only used for
the defunct -fbounded-pointers, but something else wanting it might well be
revived one day.  

> r~
> 
> 
> 
> 	* sysdeps/unix/sysv/linux/alpha/syscalls.list (semtimedop): New.
> 	Annotate some parameters.
> 	* sysdeps/unix/sysv/linux/alpha/sysdep.h (__NR_semtimedop): New.

Ok by me.