Decide what to do with /etc/rpm/platform processing

[pmatilai]

/etc/rpm/platform supposedly overrides automatic detection of arch-vendor-os, and appears to work for loading up the rpmrc tables thus, but at least all the %host* macros end up being original values in this case.
Also there's all manner of unused data fields and whatnot.

AIUI the original idea from 20+ years ago (before my time) was to have this file replace all the magic detection and compat voodoo, but what's in our tree is a very incomplete beginnings of that journey. We need to decide what to do with this stuff - either remove or fix it.