* Adjust GitHub CI to also build for riscv64
* Update base Ubuntu base for build from 18.04 LTS to 20.04 LTS as to support riscv64
* Minor: Exclude idea IDE files from git
* Workaround to compile for 32bit
This is a workaround based on [this
comment](https://github.com/antlr/antlr4/issues/2433#issuecomment-774514106).
It is not a pretty solution but it worked well on my test file.
It runs once to compile, if make fails, then it apply the fix and try
again, this is done so builds that don't need workaround don't use it
and also because the workaround is done on top of downloaded files
during ethe first compilation.
* Update README to remove ARMv7 error comment
