posix-cpu-timers: Use dedicated flag for CPU timer nanosleep

posix-cpu-timers: Use dedicated flag for CPU timer nanosleep

POSIX CPU timer nanosleep creates a k_itimer on stack and uses the sigq
pointer to detect the nanosleep case in the expiry function.

Prepare for embedding sigqueue into struct k_itimer by using a dedicated
flag for nanosleep.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/all/20241105064213.238550394@linutronix.de