Add support for the free Intel ModelSim edition

[therbom]

This pull request adds support for ModelSim - Intel FPGA Starter Edition, also known as the free edition of ModelSim that is available for Linux. It can be downloaded on this page (free Intel account required). The available features in this edition can be found here. Most notably, it claims mixed-language support, although I did not test this personally.

The following workarounds were required:

1. The free version is only available in 32 bits, so even on x86-64 we need to force OpenPiton's custom PLI library to be compiled in 32-bit mode and activate it. mkplilib has been updated accordingly.
2. The UVM library is not included in the free version, so I downloaded it manually from here and modified mktools to compile it. It's licensed under the Apache License, Version 2.0.
3. sims automatically applies the workarounds when the UVM library is missing.

I confirmed for the following runs with a single Ariane core that the simulation logs were identical with free version 2020.1 and Questa 2020.4 (Ubuntu 18.04, AMD Ryzen 7 3700X @ 4.3 GHz):

1. It takes around 7 minutes and 50 seconds before the processor finishes initialization and starts executing from the boot ROM.
2. rv64ui-p-addi.S takes 8 minutes and 20 seconds in total, according to the simulation log.
3. vvadd.riscv takes 13 minutes and 3 seconds in total, according to the simulation log.
4. accu_test.c takes 1 hour, 13 minutes and 3 seconds in total, according to the simulation log.

[therbom]

I simply unpacked uvm-1.2, but it seems that uvm-1.2/docs is 14.2 MB large. I can delete it if you want to free up some space, since I guess it is not really necessary.