anv: Use python to generate ICD json files

anv: Use python to generate ICD json files

This is more lines of code but the python is far easier to read than the
sed expressions we were using before.  Also, this allows us to pull the
API version from anv_entrypoints.py so it never gets out-of-sync.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>