Change the way Vex represents architecture variants into something

Change the way Vex represents architecture variants into something
more flexible.  Prior to this change, the type VexSubArch effectively
imposed a total ordering on subarchitecture capabilities, which was
overly restrictive.  This change moves to effectively using a bit-set,
allowing some features (instruction groups) to be supported or not
supported independently of each other.

git-svn-id: svn://svn.valgrind.org/vex/trunk@1555