No make rules for re-creating pre-built 'policy.wasm'

The recently released [prebuilt dcap 1.22 archive](https://download.01.org/intel-sgx/sgx-dcap/1.22/linux/prebuilt_dcap_1.22.tar.gz) contains a new file `./prebuilt/opa_bin/policy.wasm`

There do not appear to be any makefile rules that generate this binary file. Given the name I'm guessing that it was probably created from the `./QuoteVerification/appraisal/qal/opa_bin/qal_script.rego` file, but I'm unclear what exact `opa` command line to use to generate the WASM file from this `rego` file in a reproducible way.

Please at least document this, but preferably provide makefile rules to allow users to generate it during the build process.