arch: riscv: stacktrace: fix user thread stack bound check

According to the riscv's `arch.h`:

 +------------+ <- thread.arch.priv_stack_start
 | Guard      | } Z_RISCV_STACK_GUARD_SIZE
 +------------+
 | Priv Stack | } CONFIG_PRIVILEGED_STACK_SIZE
 +------------+ <- thread.arch.priv_stack_start +
                   CONFIG_PRIVILEGED_STACK_SIZE +
                   Z_RISCV_STACK_GUARD_SIZE

The start of the privilege stack should be:

  `thread.arch.priv_stack_start + Z_RISCV_STACK_GUARD_SIZE`

Instead of

  `thread.arch.priv_stack_start - CONFIG_PRIVILEGED_STACK_SIZE`

For the `end`, use the same equation of `top_of_priv_stack` in
the `arch_user_mode_enter()`

Signed-off-by: Yong Cong Sin <ycsin@meta.com>
(cherry picked from commit 7db18ab7)