Add initial version of risugen.
Add initial version of risugen.

This can write an ARM test image which just does the 'clean
registers' prologue and then ends the test.

hw-claudio/risu-aarch64,vivier/risu,hw-claudio/risu-aarch64,stsquad/risu,stsquad/risu,stsquad/risu,vivier/risu