[llvm] Use TargetABI to assign default-target features in getDefaultSubtargetFeatures (PR #100833)

[Paul Kirth via llvm-commits]

https://github.com/ilovepi commented:

Left some small nits and questions. This mostly seems fine, but lets see if anyone more experienced w/ the ABI features disagrees or sees an obvious foot-gun. 

Off the top of my head @topperc has done some work in this area. Maybe he can suggest another person to check in with?

https://github.com/llvm/llvm-project/pull/100833