arch: riscv: the `s0` dumped during a fatal error is not correct

[ycsin]

Describe the bug
The content of the s0 register dump during a fatal error is not the one at the time of the crash. On exception the isr.S, made a copy of the s0 register and then made some changes to the reg, but it didn't restore the s0 to the saved value before it jumps to the z_riscv_fatal_error_csf().

To Reproduce

1. Run the test added in arch: riscv: isr.S: restore s0 before jumping to z_riscv_fatal_error_csf #79096 with Zephyr v3.7-branch
2. The test will fail

Expected behavior
The test should pass

Impact
Incorrect s0 content shown in the fatal error message

[fkokosinski]

I believe this one should have been closed automaically when #79299 and #79096 were merged.