It has taken me a ridiculous amount of time to discover the following simple truth…

Since the update of net-misc/neon to 0.28.2 app-arch/rpm must be installed with the ~x86 keyword. This pulls in app-arch/rpm 4.4.6-r5

Without the ~x86 keyword, the emerge will try to compile rpm 4.4.6-r3 which is guranteed to fail.