New branch created for "available features" support

[Jim Blandy]

So a "feature" would be something like "SSE", or "MMX" with the
additional SSE registers, and then a "feature set" would be something
like "Pentium MMX" or "Pentium 3", referring to the appropriate
individual feature sets?

I understand why you want to supply base register numbers in feature
sets, rather than features; it makes it possible to share features. 
But if we're going to have other sorts of features as of yet
unimagined, they may or may not expect a base register number as a
parameter.  That is, you've tied something specific to register banks
to your <feature> entities.

I'd say, rename <feature> to <register-set>.  Introduce new,
appropriately named kinds of entities for new kinds of features.  That
way, you can look at the tag and know what sorts of parameters it
expects.