Fixes for Chisel 3.6 support

[sifive-benjamin-morse]

Related PRs / Issues

UI / API Impact

These are required to allow Firesim to upgrade to Chisel version 3.6

Verilog / AGFI Compatibility

no changes

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[davidbiancolin]

This is such a weird change, but i approved it before so, lol

[davidbiancolin]

Context for Berkeley is that these are fixes that should make the codebase compatible with 3.6 (which is what we use at SiFive). A bump to 3.6 itself is not compulsory.