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/vdev_rebuild.c

@@ -133,7 +133,7 @@ static int zfs_rebuild_scrub_enabled = 1;
 /*
  * For vdev_rebuild_initiate_sync() and vdev_rebuild_reset_sync().
  */
-static void vdev_rebuild_thread(void *arg);
+static _Noreturn void vdev_rebuild_thread(void *arg);
 
 /*
  * Clear the per-vdev rebuild bytes value for a vdev tree.
@@ -736,7 +736,7 @@ vdev_rebuild_load(vdev_t *vd)
  * Each scan thread is responsible for rebuilding a top-level vdev.  The
  * rebuild progress in tracked on-disk in VDEV_TOP_ZAP_VDEV_REBUILD_PHYS.
  */
-static void
+static _Noreturn void
 vdev_rebuild_thread(void *arg)
 {
 	vdev_t *vd = arg;