core: kernel: move spin_lock_debug.c to core/kernel

Source file spin_lock_debug.c does not contain architecture-specific code
therefore, move it from core/arch/arm/kernel to core/kernel and remove
u

core: kernel: move spin_lock_debug.c to core/kernel

Source file spin_lock_debug.c does not contain architecture-specific code
therefore, move it from core/arch/arm/kernel to core/kernel and remove
unused header thread_private.h

Signed-off-by: Marouene Boubakri <marouene.boubakri@nxp.com>
Acked-by: Jens Wiklander <jens.wiklander@linaro.org>
Reviewed-by: Etienne Carriere <etienne.carriere@linaro.org>
Acked-by: Jerome Forissier <jerome@forissier.org>

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