Assertions (and their exclusive inputs) should be guarded by `ifdefs

Our assertions should be guarded by `` `ifdef `` guards. This allows users to easily disable assertions without a recompile and also prevents assertions from being visible to synthesis- I believe yosys in particular can leave in stuff that causes problems for openroad if assertions aren't stripped.

Ideally, we'd also guard all the wires/regs that only feed assertions (or other wires/regs that only feed assertions) in those same guards to avoid "value not read" issues.