arch: riscv: IRQ stacks of secondary cores are not initialized to `0xAA` when `CONFIG_INIT_STACKS=y`

[ycsin]

Describe the bug

Currently, the IRQ stacks of secondary cores are not initialized to 0xAA when CONFIG_INIT_STACKS=y, this results in the watermark not being correctly computed by Zephyr, which can be seen by doing kernel thread stacks. This issue also exists in at least v3.5.0 and earlier.

Not sure if this has any other implications, other that the kernel thread stacks not displaying the watermark correctly.

*** Booting Zephyr OS build zephyr-v3.5.0-80-gd47129f0f7d5 ***
Hello World! qemu_riscv64_smp


uart:~$ kernel stacks 
0x80010870 idle 01                          (real size 1024):   unused  612     usage  412 / 1024 (40 %)
0x80010c00 sysworkq                         (real size 1024):   unused  668     usage  356 / 1024 (34 %)
0x800100f0 shell_uart                       (real size 3072):   unused 1268     usage 1804 / 3072 (58 %)
0x80010710 idle 00                          (real size 1024):   unused  612     usage  412 / 1024 (40 %)
0x800119b0 IRQ 00                           (real size 2048):   unused 1236     usage  812 / 2048 (39 %)
0x800121b0 IRQ 01                           (real size 2048):   unused    0     usage 2048 / 2048 (100 %)

To Reproduce

1. Check out current main branch
2. west build -b qemu_riscv64_smp -p auto -t run zephyr/samples/hello_world -- -DCONFIG_SHELL=y
3. Run shell command: kernel thread stacks
4. Check the IRQ 01++ stacks, the usage is always at 100%

Expected behavior

The usage should reflect the real watermark of the IRQ stack, which should be much lesser than 100%

Impact

z_stack_space_get will return the actual usage of secondary cores' IRQ stack

Additional context

#78053