Register refactoring part 1

This cleans up `Registers` a bit and starts implementing 64-bit support.  I haven't done the non-inline parts of `Registers.cc` yet because most of that requires the GDB interface to be straightened out first.  (And I seem to have lost the initial implementation I did when I was just trying to get rr compiling in 64-bit mode...)

I'm not really sure about 221ce65; the comment there is opaque to me and looking at the clone syscall assembly, I can't really see what clearing out ebp in that case does.  It could be the frame pointer, or we could be overwriting the 6th syscall arg?  Not clear to me.  Happy to make it poke at the 6th syscall arg instead if that's what's really required.
This commits us to an implementation where `Registers` uses `user_regs_struct` for storage. I guess that's OK since we only use this in a couple of places, making it easy to change.