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/raidz_test/raidz_test.c

@@ -840,7 +840,7 @@ static kcondvar_t sem_cv;
 static int max_free_slots;
 static int free_slots;
 
-static void
+static _Noreturn void
 sweep_thread(void *arg)
 {
 	int err = 0;

cmd/zgenhostid/zgenhostid.c

@@ -36,7 +36,7 @@
 #include <time.h>
 #include <unistd.h>
 
-static __attribute__((noreturn)) void
+static _Noreturn void
 usage(void)
 {
 	(void) fprintf(stderr,

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;

cmd/ztest/ztest.c

@@ -558,7 +558,7 @@ enum ztest_object {
 	ZTEST_OBJECTS
 };
 
-static void usage(boolean_t) __NORETURN;
+static _Noreturn void usage(boolean_t);
 static int ztest_scrub_impl(spa_t *spa);
 
 /*
@@ -622,7 +622,7 @@ static void sig_handler(int signo)
 
 char *fatal_msg;
 
-static __attribute__((noreturn)) __attribute__((format(printf, 2, 3))) void
+static __attribute__((format(printf, 2, 3))) _Noreturn void
 fatal(int do_perror, char *message, ...)
 {
 	va_list args;
@@ -6993,7 +6993,7 @@ ztest_resume(spa_t *spa)
 	(void) zio_resume(spa);
 }
 
-static void
+static _Noreturn void
 ztest_resume_thread(void *arg)
 {
 	spa_t *spa = arg;
@@ -7019,7 +7019,7 @@ ztest_resume_thread(void *arg)
 	thread_exit();
 }
 
-static void
+static _Noreturn void
 ztest_deadman_thread(void *arg)
 {
 	ztest_shared_t *zs = arg;
@@ -7097,7 +7097,7 @@ ztest_execute(int test, ztest_info_t *zi, uint64_t id)
 		    (double)functime / NANOSEC, zi->zi_funcname);
 }
 
-static void
+static _Noreturn void
 ztest_thread(void *arg)
 {
 	int rand;