[Patch]Extend GAS arm_feature_set struct to provide more available bits

[Terry Guo]

Hi Nick,

Patch is committed. I tried several different approaches last week.
Failed to find a suitable one. I have to stick with my old patch with
your comments on macro. Given we have 30 more available bits now, I
think it is ok for next decades. So it is less likely we will need to
extend it in near future. Thanks for your review and comments.

BR,
Terry