I'm not aware of an official way using the CLI/API, unless you want to populate a table of routing-engine part numbers and map those to CPU architecture.
Another method likely to work is run uname -a and match on ppc or amd64:
jsw@mx240> start shell
% uname -a
FreeBSD mx240 JNPR-11.0-20180730.2cd3a6e_buil FreeBSD JNPR-11.0-20180730....