No it's not using hwcaps. That would only allow optimization of code in shared libraries, would be irritating to implement in a way that didn't require touching each package that includes shared libraries and would (depending on details) waste a bunch of space on every users system. I think hwcaps would only make sense for a small number of shared libraries if at all, not a system wide thing.
I am probably going to be the one implementing this and I don't know what I am going to do yet! At the very least we need the failure mode to be better (currently you get an OOPS when the init from the initrd dies due to an illegal instruction exception)