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

tests/zfs-tests/cmd/mkbusy/mkbusy.c

@@ -31,14 +31,14 @@
 #include <string.h>
 
 
-static __attribute__((noreturn)) void
+static _Noreturn void
 usage(char *progname)
 {
 	(void) fprintf(stderr, "Usage: %s <dirname|filename>\n", progname);
 	exit(1);
 }
 
-static __attribute__((noreturn)) void
+static _Noreturn void
 fail(char *err)
 {
 	perror(err);

tests/zfs-tests/cmd/mkfile/mkfile.c

@@ -44,7 +44,7 @@
 
 #define	FILE_MODE	(S_ISVTX + S_IRUSR + S_IWUSR)
 
-static void usage(void) __attribute__((noreturn));
+static _Noreturn void usage(void);
 
 int
 main(int argc, char **argv)