riscv: smp: fix secondary cpus' initial stack

[npitre]

Z_THREAD_STACK_BUFFER() must not be used here. This is meant for stacks
defined with K_THREAD_STACK_ARRAY_DEFINE() whereas in this case we are
given a stack created with K_KERNEL_STACK_ARRAY_DEFINE().

If CONFIG_USERSPACE=y then K_THREAD_STACK_RESERVED gets defined with
a bigger value than K_KERNEL_STACK_RESERVED. Then Z_THREAD_STACK_BUFFER()
returns a pointer that is more advanced than expected, resulting in a
stack pointer outside its actual stack area and therefore memory
corruption ensues.

Fixes: #50465

[stephanosio]

It seems we have the same issue on a few other archs as well:

zephyr/soc/xtensa/esp32/esp32-mp.c

Line 238 in 51af614

sr.stack_top = Z_THREAD_STACK_BUFFER(stack) + sz;

zephyr/soc/xtensa/intel_adsp/common/multiprocessing.c

Line 140 in 0687a61

z_mp_stack_top = Z_THREAD_STACK_BUFFER(stack) + sz;

zephyr/arch/arc/core/arc_smp.c

Line 53 in 83d73f6

arc_cpu_sp = Z_THREAD_STACK_BUFFER(stack) + sz;

cc @dcpleung @ruuddw

Tracked by #50467 and #50468