Update rvfi_mem interface.

Created by: silabs-oivind

Fix 3 issues:

1. Transfers could be missed if there's backpressure from WB when lsu_data_trans_valid is asserted.
2. If the 2nd half of a split misaligned transfer is blocked, it would still be reported on rvfi_mem.
3. rvfi_mem_rdata was not correct for split misaligned transfers. Only rdata from 1st transfer would be reported on rvfi_mem

The bugs fixed in this PR were found by assertions written for https://github.com/openhwgroup/cv32e40x/issues/648 The PR for these assertions will follow.