rvfi_tracer.sv: Terminate tests on write-into-tohost iso. ECALL instruction.

Created by: zchamski

This PR changes the simulation termination condition from "execute an ECALL instruction" to "write 0b........1 at tohost memory address".

• The value written into tohost must have its least-significant bit set.
• The bits above the LSbit contain the return value of the test.
• The execution of an ecall instruction does not terminate the simulation anymore. Instead, the execution of the ecall instruction will be handled as implemented, including the side effects in the environment.

Supersedes https://github.com/openhwgroup/cva6/pull/1064 (rebased, different source branch name).