Fix GitHub workflows
Switching to Clang with commit 66bff8e ("Convert object_defs.py
to auto-generated file") caused the GitHub build/release workflows
to stop working due to the new requirements for Clang, LLVM, and
the Clang Python bindings.
Fixing this requires adding new GitHub actions to install LLVM and
Clang, configure the build environment and make some code changes
to allow for emulator to compile cleanly.