arch: riscv: update PMP setting to privileged mode for fault handler

When RISCV_ALWAYS_SWITCH_THROUGH_ECALL is enabled, do_swap() enables PMP
checking in is_kernel_syscall.
If a user thread violates memory protection and do_swap() is called from
the fault handler, a PMP error occurs because the thread is in privileged
mode but still using the old user mode PMP setting.

Update the PMP setting to privileged mode for fault handler.
This also enables the stack guard for user thread's privileged stack in
fault handler.

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