fix tracer printing that deviates from riscv assembly format

Created by: silabs-robin

I know this tracer is planned to be obsoleted by the rvfi tracer, but in the meantime there are 3 issues that are a bit in my way. This PR does not necessarily need to be accepted, because one can just use the CV_CORE_PATH variable in core-v-verif, but it would be nice to have the actual fixes in place.

Problem 1 is "nop". This is a pseudo instruction and this is exceptional in the tracer because there are no other pseudoinstructions. And it causes problems when diffing against other execution logs.

Problem 2 is that "fencei" is actually named "fence.i" (as seen in e.g. objdump).

Problem 3 is that "shamt" was printed wrong, because it essentially included "funct7".