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-11.0-20180730.2cd3a6e_builder_stable_11 #0 r356482+2cd3a6e(HEAD)-dirty: Mon Jul 30 08:20:09 PDT 2018 builder@feyrith.juniper.net:/volume/build/junos/occam/gcc/freebsd/stable_11/20180725.182919_builder_stable_11.2cd3a6e/obj/amd64/juniper/kernels/JNPR-AMD64-PRD/kernel amd64
jsw@mx80> start shell
% uname -a
JUNOS mx80 16.1R5.7 JUNOS 16.1R5.7 #0: 2017-08-18 17:36:30 UTC builder@toyotama.juniper.net:/volume/build/junos/16.1/release/16.1R5.7/obj/powerpc/junos/bsd/kernels/JUNIPER-PPC/kernel powerpc