testing/yosys: upgrade to 0.42

https://github.com/YosysHQ/yosys/releases/tag/yosys-0.42

Note: This failed to link with clang/LLVM, but worked out of the box
      with GCC. So, I just changed the toolchain to GCC.
Also note: The unit test segfaulted with LTO. Hence, LTO was disabled.