Fix simulation mismatch between VCS and QuestaSim

[Xeratec]

During simulations with a custom GitLab branch, I experienced simulation mismatches between QuestaSim and VCS.
Strangely, the snitch_addr_demux module did work with NumRules larger than one. However, with NumRules equal to one, the slave_select signal was not correctly assigned (as shown in the figures below).

Additionally, this PR makes sure that VCS uses the custom compiler specified via the CC and CCX environment variables. This modification was required on my IIS workstation and adds additional flexibility.

Changelog

Changed

• Use custom compiler for VCS specified with CC and CCX environment variable

Fixed

• Fix type issue in snitch_addr_demu

Checklist

• Automated tests pass
• Check modifications with Synopsys SpyGlass
• Changelog updated
• Code style guideline is observed

Images

Correct Behavior in QuestaSim

Incorrect Behavior in QuestaSim

Origin of the Mismatch

[SamuelRiedel]

LGTM. Thanks :)