I encountered a project that uses llvm that passes "generic" by
default. While I could fix that project, I wouldn't be surprised
if other projects did something similar. So it seems like
a good idea to be defensive.
I've also added validation of the 64Bit feature against the
triple so that we can catch a mismatched CPU before failing in
a mysterious way. We can make it pretty far in isel because we
calculate XLenVT from the triple and use that to set up the legal