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

lib/libuutil/uu_pname.c

@@ -40,7 +40,7 @@
 
 static const char *pname;
 
-static _Noreturn void
+static __attribute__((noreturn)) void
 uu_die_internal(int status, const char *format, va_list alist);
 
 int uu_exit_ok_value = EXIT_SUCCESS;
@@ -110,7 +110,7 @@ uu_warn(const char *format, ...)
 	va_end(alist);
 }
 
-static __attribute__((format(printf, 2, 0))) _Noreturn void
+static __attribute__((format(printf, 2, 0))) __attribute__((noreturn)) void
 uu_die_internal(int status, const char *format, va_list alist)
 {
 	uu_warn_internal(errno, format, alist);

lib/libzpool/kernel.c

@@ -633,7 +633,7 @@ __dprintf(boolean_t dprint, const char *file, const char *func,
 static char ce_prefix[CE_IGNORE][10] = { "", "NOTICE: ", "WARNING: ", "" };
 static char ce_suffix[CE_IGNORE][2] = { "", "\n", "\n", "" };
 
-void
+__attribute__((noreturn)) void
 vpanic(const char *fmt, va_list adx)
 {
 	(void) fprintf(stderr, "error: ");
@@ -643,7 +643,7 @@ vpanic(const char *fmt, va_list adx)
 	abort();	/* think of it as a "user-level crash dump" */
 }
 
-void
+__attribute__((noreturn)) void
 panic(const char *fmt, ...)
 {
 	va_list adx;

lib/libzpool/taskq.c

@@ -211,7 +211,7 @@ taskq_wait_outstanding(taskq_t *tq, taskqid_t id)
 	taskq_wait(tq);
 }
 
-static _Noreturn void
+static __attribute__((noreturn)) void
 taskq_thread(void *arg)
 {
 	taskq_t *tq = arg;