arch: riscv: isr.S: restore s0 before jumping to z_riscv_fatal_error_csf

Restore the s0 we saved early in ISR entry so it shows up
properly in the CSF.

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