Like o11c says, that's setting everyone up for a bad time. If the heterogenous cores are similar, but don't all support all the instructions, it's too hard to use. You can build legacy instructions in a space optimized way though, but there's no reason not to do that for the high performance cores too --- if they're legacy instructions, one expects them not to run often and perf doesn't matter that much.

Intel dropped their x86-S proposal; but I guess something like that could work for low power cores. If you provide a way for a 64-bit OS to start application processors directly in 64-bit mode, you could setup low power cores so that they could only run in 64-bit mode. I'd be surprised if the juice is worth the squeeze, but it'd be reasonable --- it's pretty rare to be outside 64-bit mode, and systems that do run outside 64-bit mode probably don't need all the cores on a modern processor. If you're running in a 64-bit OS, it knows which processes are running in 32-bit mode, and could avoid scheduling them on reduced functionality cores; If you're running a 32-bit OS, somehow or another the OS needs to not use those cores... either the ACPI tables are different and they don't show up for 32-bit, init fails and the OS moves on, or the there is a firmware flag to hide them that must be set before running a 32-bit OS.