selftests: kselftest: Mark functions that unconditionally call exit() as __noreturn

After commit 6d029c25 ("selftests/timers/posix_timers: Reimplement
check_timer_distribution()"), clang warns:

  tools/testing/selftests/timers/../kselftest.h:398:6: warning: variable 'major' is used uninitialized whenever '||' condition is true [-Wsometimes-uninitialized]
    398 |         if (uname(&info) || sscanf(info.release, "%u.%u.", &major, &minor) != 2)
        |             ^~~~~~~~~~~~
  tools/testing/selftests/timers/../kselftest.h:401:9: note: uninitialized use occurs here
    401 |         return major > min_major || (major == min_major && minor >= min_minor);
        |                ^~~~~
  tools/testing/selftests/timers/../kselftest.h:398:6: note: remove the '||' if its condition is always false
    398 |         if (uname(&info) || sscanf(info.release, "%u.%u.", &major, &minor) != 2)
        |             ^~~~~~~~~~~~~~~
  tools/testing/selftests/timers/../kselftest.h:395:20: note: initialize the variable 'major' to silence this warning
    395 |         unsigned int major, minor;
        |                           ^
        |                            = 0

This is a false positive because if uname() fails, ksft_exit_fail_msg()
will be called, which unconditionally calls exit(), a noreturn function.
However, clang does not know that ksft_exit_fail_msg() will call exit() at
the point in the pipeline that the warning is emitted because inlining has
not occurred, so it assumes control flow will resume normally after
ksft_exit_fail_msg() is called.

Make it clear to clang that all of the functions that call exit()
unconditionally in kselftest.h are noreturn transitively by marking them
explicitly with '__attribute__((__noreturn__))', which clears up the
warning above and any future warnings that may appear for the same reason.

Fixes: 6d029c25 ("selftests/timers/posix_timers: Reimplement check_timer_distribution()")
Reported-by: John Stultz <jstultz@google.com>
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Shuah Khan <skhan@linuxfoundation.org>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20240411-mark-kselftest-exit-funcs-noreturn-v1-1-b027c948f586@kernel.org
Closes: https://lore.kernel.org/all/20240410232637.4135564-2-jstultz@google.com/