makefile: add support for built-in ghdl-yosys-plugin

[umarcor]

ghdl-yosys-plugin provides an open source VHDL frontend based on GHDL, which allows Yosys to handle VHDL 1987, 1993, 2000 or 2008. There are two options for building ghdl-yosys-plugin:

• Build as a module (shared library)
• Build as part of yosys (not recommended)

On Windows, Yosys does not support loading plugins as external shared libraries, they need to be built-in. Therefore, using the second (not recommended) approach is required. It involves patching Yosys' Makefile: yosys.diff. That patch is harmless and having it upstreamed here can make building easier for less experienced users. After merging this PR, the build procedure would be:

  git clone https://github.com/YosysHQ/yosys
  git clone https://github.com/ghdl/ghdl-yosys-plugin

  cd yosys
  mv ../ghdl-yosys-plugin/src frontends/ghdl
  
  make config-*
  
  echo "ENABLE_GHDL=1" >> Makefile.conf
  # Optionally, set the prefix:
  # echo "GHDL_DIR=${MINGW_PREFIX}" >> Makefile.conf

  make

/cc @tgingold