arch: riscv: handle PMP setting for _Fault handler

[jimmyzhe]

When CONFIG_RISCV_ALWAYS_SWITCH_THROUGH_ECALL is enabled, do_swap() from _Fault triggers a PMP error again if the fault is caused by a PMP violation. This PR handles the PMP settings for faults caused by PMP violations:

1. For kernel thread stack overflow:
   Remove the previous thread's PMP configuration.

2. For user thread violates memory protection:
   Configure the PMP for the thread's privileged stack.

Although this bug occurs only when CONFIG_RISCV_ALWAYS_SWITCH_THROUGH_ECALL is enabled, I think these fixes are also compatible when CONFIG_RISCV_ALWAYS_SWITCH_THROUGH_ECALL is disabled.

Fixes: #75959

[npitre]

A few comments:

• Please elaborate more in the commit message for "protect privileged stack
  for fault handler" with a list of events illustrating the issue. I still
  don't see where the problem is.

• I imagine the second patch is a consequence of the first? Please say so in
  the commit log. Otherwise it looks like you add stack guard just to remove
  it later.

• Couldn't z_riscv_pmp_stackguard_disable() be implemented by simply clearing
  MSTATUS_MPRV?

[jimmyzhe]

@npitre, thank you for your feedback.

Please elaborate more in the commit message for "protect privileged stack
for fault handler" with a list of events illustrating the issue. I still
don't see where the problem is.

The first commit majorly change PMP setting to privileged mode for faults from user mode.
Protect privileged stack just the side effect of this fix.
I have update the commit message to make it clearer.

I imagine the second patch is a consequence of the first? Please say so in
the commit log. Otherwise it looks like you add stack guard just to remove
it later.

These commits are independent to fix 2 different cases:

1. user thread violates PMP memory protection
   The first commit changes PMP setting to privileged mode for fault triggered from user mode.
2. kernel thread triggers PMP stack guard
   The second commit removes the thread's PMP setting for fault triggered by stack overflow.

Couldn't z_riscv_pmp_stackguard_disable() be implemented by simply clearing
MSTATUS_MPRV?

Disabling mstatus.mprv is not enough because do_swap() enables mstatus.mprv in is_kernel_syscall when CONFIG_RISCV_ALWAYS_SWITCH_THROUGH_ECALL is enabled.
The previous stack overflow PMP setting triggers an error during the do_swap() call from the fault handler.