util: Replace all POSIX arch busy_waits with Z_SPIN_DELAY

A new Z_SPIN_DELAY() macro has been added which
can be used to reduce a bit the amount of noise
due to the POSIX arch need to break busy loops with
k_busy_wait().
Use it.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>