arch: irq: Use Z_ISR_DECLARE_DIRECT for direct ISR declaration.

This commit changes the way how ARCH_IRQ_DIRECT_CONNECT is defined.
Now it uses Z_ISR_DECLARE_DIRECT internally.
That is a requirement for local isr declaration.

Signed-off-by: Radosław Koppel <radoslaw.koppel@nordicsemi.no>