Use _Noreturn (C11; GNU89) properly

A function that returns with no value is a different thing from a
function that doesn't return at all.  Those are two orthogonal
concepts, commonly confused.

pthread_create(3) expects a pointer to a start routine that has a
very precise prototype:

    void *(*start_routine)(void *);

However, other thread functions, such as kernel ones, expect:

    void (*start_routine)(void *);

Providing a different one is incorrect, and has only been working
because the ABIs happen to produce a compatible function.

We should use '_Noreturn void', since it's the natural type, and
then provide a '_Noreturn void *' wrapper for pthread functions.

For consistency, replace most cases of __NORETURN or
__attribute__((noreturn)) by _Noreturn.  _Noreturn is understood
by -std=gnu89, so it should be safe to use everywhere.

Ref: https://github.com/openzfs/zfs/pull/13110#discussion_r808450136
Ref: https://software.codidact.com/posts/285972
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Co-authored-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Signed-off-by: Alejandro Colomar <alx.manpages@gmail.com>
Closes #13120

include/os/linux/spl/sys/thread.h

@@ -62,7 +62,7 @@ typedef void (*thread_func_t)(void *);
 extern kthread_t *__thread_create(caddr_t stk, size_t  stksize,
     thread_func_t func, const char *name, void *args, size_t len, proc_t *pp,
     int state, pri_t pri);
-extern void __thread_exit(void);
+extern __attribute__((noreturn)) void __thread_exit(void);
 extern struct task_struct *spl_kthread_create(int (*func)(void *),
     void *data, const char namefmt[], ...);