[PATCH] Replace the bitfield of SubtargetFeatures with a BitVector

[Tim Northover]

> Tim, do you want me to make this change to the AArch64 part as is?
> You said you may have another idea.

I thought I'd already applied the AArch64 change (there's no more
Features in the union, so probably no refactoring needed for you). But
you'll probably have quite a few merge conflicts to solve before
committing.

Cheers.

Tim.