arch: riscv: optionally stores a pointer to csf in `struct arch_esf`

The callee-saved-registers can be helpful to debug the state of
a core upon an exception, however, currently there's no way to
access that information in user-implemented
`k_sys_fatal_error_handler()`, even though the csf is already stored
in the stack.

This patch conditionally add a `csf` member in the `arch_esf` when
`CONFIG_EXTRA_EXCEPTION_INFO=y`*, which the `_isr_wrapper` would update
when a fatal error occurs before invoking `z_riscv_fatal_error_csf()`.

Functions such as `k_sys_fatal_error_handler()` would then be able
to access the callee-saved-registers at the time of exception via
`esf->csf`.

* For SoCs that select `RISCV_SOC_HAS_ISR_STACKING`, the
  `SOC_ISR_STACKING_ESF_DECLARE` has to include the `csf` member,
  otherwise the build would fail.

Signed-off-by: Yong Cong Sin <ycsin@meta.com>
Signed-off-by: Yong Cong Sin <yongcong.sin@gmail.com>