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/txg.c

@@ -108,8 +108,8 @@
  * now transition to the syncing state.
  */
 
-static void txg_sync_thread(void *arg);
-static void txg_quiesce_thread(void *arg);
+static _Noreturn void txg_sync_thread(void *arg);
+static _Noreturn void txg_quiesce_thread(void *arg);
 
 int zfs_txg_timeout = 5;	/* max seconds worth of delta per txg */
 
@@ -514,7 +514,7 @@ txg_has_quiesced_to_sync(dsl_pool_t *dp)
 	return (tx->tx_quiesced_txg != 0);
 }
 
-static void
+static _Noreturn void
 txg_sync_thread(void *arg)
 {
 	dsl_pool_t *dp = arg;
@@ -605,7 +605,7 @@ txg_sync_thread(void *arg)
 	}
 }
 
-static void
+static _Noreturn void
 txg_quiesce_thread(void *arg)
 {
 	dsl_pool_t *dp = arg;