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

module/zfs/arc.c

@@ -928,7 +928,7 @@ static unsigned long l2arc_rebuild_blocks_min_l2size = 1024 * 1024 * 1024;
 
 /* L2ARC persistence rebuild control routines. */
 void l2arc_rebuild_vdev(vdev_t *vd, boolean_t reopen);
-static void l2arc_dev_rebuild_thread(void *arg);
+static _Noreturn void l2arc_dev_rebuild_thread(void *arg);
 static int l2arc_rebuild(l2arc_dev_t *dev);
 
 /* L2ARC persistence read I/O routines. */
@@ -9706,7 +9706,7 @@ l2arc_hdr_limit_reached(void)
  * This thread feeds the L2ARC at regular intervals.  This is the beating
  * heart of the L2ARC.
  */
-static void
+static _Noreturn void
 l2arc_feed_thread(void *unused)
 {
 	(void) unused;
@@ -10145,7 +10145,7 @@ l2arc_spa_rebuild_start(spa_t *spa)
 /*
  * Main entry point for L2ARC rebuilding.
  */
-static void
+static _Noreturn void
 l2arc_dev_rebuild_thread(void *arg)
 {
 	l2arc_dev_t *dev = arg;