More expressive feature syntax

[calebzulawski]

Architectures currently support an expansion:
[x86|x86_64] expands to both x86 and x86_64.

It might be useful to add this syntax to features as well:
x86+[sse|sse+sse2] expands to x86+sse and x86+sse+sse2.

It might also be useful to create some sort of permutation syntax:
x86+[sse,popcnt] expands to x86+sse, x86+popcnt and x86+sse+popcnt?

Since many (but definitely not all) features imply the existence of other features, maybe there should be a way to specify sequential features rather than permutation:
x86+{sse+sse2} expands to x86+sse, x86+sse+sse2 (not sold on this syntax, there might be a more obvious way of denoting this).

[calebzulawski]

This is incompatible with how targets are treated now, so closing for now.