Add support for getenvval as wrapper for Verilator's getenv

Created by: Karl-Han

As suggested in #723 (closed) by europe2024 and demand for support of getenv in Verilator, this commit adds support for getenv through DPI.

The reason why it is not using getenv directly is that there are some problems with the function signature in C and C++. So it is necessary to add wrapper around getenv to explicitly specify it is using C++ version getenv.