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_trim.c

@@ -834,7 +834,7 @@ vdev_trim_range_add(void *arg, uint64_t start, uint64_t size)
  * by its ms_allocatable.  While a metaslab is undergoing trimming it is
  * not eligible for new allocations.
  */
-static void
+static _Noreturn void
 vdev_trim_thread(void *arg)
 {
 	vdev_t *vd = arg;
@@ -1175,7 +1175,7 @@ vdev_trim_range_verify(void *arg, uint64_t start, uint64_t size)
  * N.B. This behavior is different from a manual TRIM where a thread
  * is created for each leaf vdev, instead of each top-level vdev.
  */
-static void
+static _Noreturn void
 vdev_autotrim_thread(void *arg)
 {
 	vdev_t *vd = arg;
@@ -1514,7 +1514,7 @@ vdev_autotrim_restart(spa_t *spa)
 		vdev_autotrim(spa);
 }
 
-static void
+static _Noreturn void
 vdev_trim_l2arc_thread(void *arg)
 {
 	vdev_t		*vd = arg;