Corrected an issue with corev-dv compilation using vsim & regress improvements

Created by: XavierAubert

I had an issue where vsim crashes while running multiple corev-dv instruction generation in parallel. After some code digging, I found that for non-corev-dv simulations, vlog/vopt combo is not called in regression environment thanks to COMP=0 parameter. This behavior was not implemented for corev-dv comp/opt/simulation, so here it is (commit 2cc69ed8), and it solves my issue.

While I was looking after this, I also saw two things I considered issues for the corev-dv commands, that includes:

• Compilation of the whole core just for generating the assembly files
• vsim flags used for corev-dv compilation/simulations are the same for the "real" simulation run

So, inside the same commit 2cc69ed8, I removed core compilaton & added 2 distinct variables. Let me know if there is any specific reason to compile the core files for a testbench that doesn't stimulate it, I may have missed something...

The second commit of the PR allows to use the precmd field of cv_regress script to generate in one unique simulation all assembly files for a same test, it allows reducing compilation time/CPU usage.