arch: riscv: fill all IRQ stacks with 0xAA

Fill the memory of all CPU's IRQ stack with 0xAA on init, so
that `z_stack_space_get` can calculate the remaining space
correctly.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>
Signed-off-by: Yong Cong Sin <ycsin@meta.com>
Signed-off-by: Yong Cong Sin <yongcong.sin@gmail.com>