Enable new multithreading support in Verilator 4

[jbush001]

Use --threads flag, need to test for correct operation, produce profile results to characterize speedup.

[jbush001]

Could use the ProcessorCount cmake module to automatically determine number of threads: https://cmake.org/cmake/help/latest/module/ProcessorCount.html

[jbush001]

The CMakeLists.txt file in hardware would also need be modified to update the minimum supported version to one with multithreading support.

[jbush001]

Need to do some performance measurements and regression tests before enabling by default.

[jbush001]

Running a randomized cosimulation test (3.1 GHz Intel Core i7, 2 cores, 4 threads)

1 thread - 18s
2 threads - 16s
4 threads - 27s

[jbush001]

ProcessorCount returns 4 on my machine, even though system info reports two cores. This is presumably the number of threads. Using this value is slower than 1 thread.