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_removal.c

@@ -140,7 +140,7 @@ int zfs_removal_suspend_progress = 0;
 
 #define	VDEV_REMOVAL_ZAP_OBJS	"lzap"
 
-static void spa_vdev_remove_thread(void *arg);
+static _Noreturn void spa_vdev_remove_thread(void *arg);
 static int spa_vdev_remove_cancel_impl(spa_t *spa);
 
 static void
@@ -1589,7 +1589,7 @@ spa_remove_max_segment(spa_t *spa)
  * TXG have completed (see spa_txg_zio) and writes the new mappings to disk
  * (see vdev_mapping_sync()).
  */
-static void
+static _Noreturn void
 spa_vdev_remove_thread(void *arg)
 {
 	spa_t *spa = arg;