GCC's attribute 'target', in addition to being an optimization hint,
also allows function multiversioning. We currently have the former
implemented, this is the latter's implementation.
Note that it ends up having to permit redefinition of functions so
that they can all be emitted. Additionally, all versions of the function
must be emitted, so this also manages that.
Function templates are NOT supported (not supported in GCC either).
Options on how to split this patch up would also be particularly solicited,
since this IS a large patch.