riscv: fix IRQ stack guard location

_current_cpu->irq_stack is not yet initialized when this is executed on
CPU 0. Also the guard area is outside of CONFIG_ISR_STACK_SIZE now
e.g. it is within the K_KERNEL_STACK_RESERVED area at the start of
the buffer. So simply use z_interrupt_stacks[] directly instead.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>