arch: riscv: only use z_riscv_fatal_error_csf if CONFIG_EXCEPTION_DEBUG

Use `z_riscv_fatal_error_csf` that expects the
callee-saved-registers pointer only if `CONFIG_EXCEPTION_DEBUG`
is enabled, otherwise use `z_riscv_fatal_error`, as there can
be garbage in the `a2`.

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