rvfi_tracer.sv: Use correct width for 'tohost' address.

Created by: zchamski

Fix the width of tohost address representation in rvfi_tracer.sv to match the width of the RVFI mem_paddr field:

• in 64b mode physical addresses are 56 bits wide (PLEN == 56);
• in 32b mode physical addresses are 34 bits wide (PLEN == 34), meaning that they are wider than XLEN.

This modification is intended to resolve the failures in gate simulations seen with the original test termination PR (#1075) in both 32 and 64 bit configurations.

To keep the code clean, this PR makes TOHOST_ADDRESS always PLEN bits wide, forcing the correct bit width in the conversion of the corresponding plusarg.