This doesn't change anything for amd64 and i386 and very little for aarch64 (aarch64 will be printed instead of arm64), so I'm not sure what breakage you think about

One would be Perl's uname(): https://github.com/Perl/perl5/blob/blead/ext/POSIX/POSIX.xs#L3349. This causes e.g. OpenSSL's build system to be explicitly told what architecture to build (see security/openssl port).
Another would be Firefox's generation of default UA: https://searchfox.org/mozilla-central/source/netwerk/protocol/http/nsHttpHandler.cpp#931

I will test it on my own hardware.
If you want to do it on your own, there are two options:

• you can test a full VM with qemu-system (will be slow),
• or you can use native POWER hardware (bare-metal, VM, could be remote since there are providers that offer free POWER VM for open source developers).