arch: riscv: fatal: always print mcause & mtval

Relocate the logging of mcause & mtval from `_Fault` to
`z_riscv_fatal_error_csf` so that they are always printed
upon exception.

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