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/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;