arch: riscv: remove PMP stack guard for stack overflow handler

When RISCV_ALWAYS_SWITCH_THROUGH_ECALL is enabled, do_swap() enables PMP
checking in is_kernel_syscall.
If the PMP stack guard is triggered and do_swap() is called from the
fault handler, a PMP error occurs because the stack usage violates the
previous PMP setting.

Remove the stack guard setting during a stack overflow handler to allow
enabling PMP checking safely in fault handler.

Signed-off-by: Jimmy Zheng <jimmyzhe@andestech.com>