Switch from _Noreturn to __attribute__((noreturn))

Parts of the Linux kernel build system struggle with _Noreturn.  This
results in the following warnings when building on RHEL 8.5, and likely
other environments.  Switch to using the __attribute__((noreturn)).

  warning: objtool: dbuf_free_range()+0x2b8:
    return with modified stack frame
  warning: objtool: dbuf_free_range()+0x0:
    stack state mismatch: cfa1=7+40 cfa2=7+8
  ...
  WARNING: EXPORT symbol "arc_buf_size" [zfs.ko] version generation
    failed, symbol will not be versioned.
  WARNING: EXPORT symbol "spa_open" [zfs.ko] version generation
    failed, symbol will not be versioned.
  ...

Additionally, __thread_exit() has been renamed spl_thread_exit() and
made a static inline function.  This was needed because the kernel
will generate a warning for symbols which are __attribute__((noreturn))
and then exported with EXPORT_SYMBOL.

While we could continue to use _Noreturn in user space I've also
switched it to __attribute__((noreturn)) purely for consistency
throughout the code base.

Reviewed-by: Ryan Moeller <freqlabs@FreeBSD.org>
Reviewed-by: Brian Atkinson <batkinson@lanl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #13238

cmd/raidz_test/raidz_test.c

@@ -839,7 +839,7 @@ static kcondvar_t sem_cv;
 static int max_free_slots;
 static int free_slots;
 
-static _Noreturn void
+static __attribute__((noreturn)) void
 sweep_thread(void *arg)
 {
 	int err = 0;

cmd/zfs/zfs_main.c

@@ -499,7 +499,7 @@ usage_prop_cb(int prop, void *cb)
  * that command.  Otherwise, iterate over the entire command table and display
  * a complete usage message.
  */
-static _Noreturn void
+static __attribute__((noreturn)) void
 usage(boolean_t requested)
 {
 	int i;

cmd/zgenhostid/zgenhostid.c

@@ -36,7 +36,7 @@
 #include <time.h>
 #include <unistd.h>
 
-static _Noreturn void
+static __attribute__((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 _Noreturn void
+static __attribute__((noreturn)) void
 usage(void)
 {
 	(void) fprintf(stderr,
@@ -87,7 +87,7 @@ usage(void)
 }
 
 
-static __attribute__((format(printf, 3, 4))) _Noreturn void
+static __attribute__((format(printf, 3, 4))) __attribute__((noreturn)) void
 fatal(spa_t *spa, void *tag, const char *fmt, ...)
 {
 	va_list ap;

cmd/zpool/zpool_main.c

@@ -518,7 +518,7 @@ print_vdev_prop_cb(int prop, void *cb)
  * that command.  Otherwise, iterate over the entire command table and display
  * a complete usage message.
  */
-static _Noreturn void
+static __attribute__((noreturn)) void
 usage(boolean_t requested)
 {
 	FILE *fp = requested ? stdout : stderr;

cmd/ztest/ztest.c

@@ -558,7 +558,7 @@ enum ztest_object {
 	ZTEST_OBJECTS
 };
 
-static _Noreturn void usage(boolean_t);
+static __attribute__((noreturn)) void usage(boolean_t requested);
 static int ztest_scrub_impl(spa_t *spa);
 
 /*
@@ -622,7 +622,7 @@ static void sig_handler(int signo)
 
 char *fatal_msg;
 
-static __attribute__((format(printf, 2, 3))) _Noreturn void
+static __attribute__((format(printf, 2, 3))) __attribute__((noreturn)) void
 fatal(int do_perror, char *message, ...)
 {
 	va_list args;
@@ -831,7 +831,7 @@ fini_options(void)
 	short_opts = NULL;
 }
 
-static void
+static __attribute__((noreturn)) void
 usage(boolean_t requested)
 {
 	char option[80];
@@ -6994,7 +6994,7 @@ ztest_resume(spa_t *spa)
 	(void) zio_resume(spa);
 }
 
-static _Noreturn void
+static __attribute__((noreturn)) void
 ztest_resume_thread(void *arg)
 {
 	spa_t *spa = arg;
@@ -7020,7 +7020,7 @@ ztest_resume_thread(void *arg)
 	thread_exit();
 }
 
-static _Noreturn void
+static __attribute__((noreturn)) void
 ztest_deadman_thread(void *arg)
 {
 	ztest_shared_t *zs = arg;
@@ -7098,7 +7098,7 @@ ztest_execute(int test, ztest_info_t *zi, uint64_t id)
 		    (double)functime / NANOSEC, zi->zi_funcname);
 }
 
-static _Noreturn void
+static __attribute__((noreturn)) void
 ztest_thread(void *arg)
 {
 	int rand;