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

cmd/zhack/zhack.c

@@ -57,7 +57,7 @@ static importargs_t g_importargs;
 static char *g_pool;
 static boolean_t g_readonly;
 
-static __attribute__((noreturn)) void
+static _Noreturn void
 usage(void)
 {
 	(void) fprintf(stderr,
@@ -87,7 +87,7 @@ usage(void)
 }
 
 
-static __attribute__((noreturn)) __attribute__((format(printf, 3, 4))) void
+static __attribute__((format(printf, 3, 4))) _Noreturn void
 fatal(spa_t *spa, void *tag, const char *fmt, ...)
 {
 	va_list ap;