aarch64: use macro trickery to automate feature array size replication

aarch64: use macro trickery to automate feature array size replication

There are quite a few macros that need to be changed when we need to
increase the number of words in the features data structure.  With
some macro trickery we can automate most of this so that a single
macro needs to be updated.

With C2X we could probably do even better by using recursion, but this
is still a much better situation than we had previously.

A static assertion is used to ensure that there is always enough space
in the flags macro for the number of feature bits we need to support.