riscv: pmp: enable stackguard without multithreading

Without multithreading only two stacks present: ISR and main.
As any stack they also could overflow, so it make sense to add stack
guard for them also.

Remove stack guard dependency on multithreading and mark
`Z_RISCV_STACK_GUARD_SIZE` bytes at the beginning of stack as read-only
region with PMP entry.

Signed-off-by: Volodymyr Fialko <vfialko@marvell.com>