Permit attribute 'used' with 'target' multiversioning.

This adds infrastructure for a multiversioning whitelist, plus adds
'used' to the allowed list with 'target'.  The behavior here mirrors the
implementation in GCC, where 'used' only applies to the single
declaration and doesn't apply to the ifunc or resolver.

This is not being applied to cpu_dispatch and cpu_specific, since the
rules are more complicated for cpu_specific, which emits multiple
symbols. Additionally, the author isn't currently aware of uses in the
wild of this combination, but is aware of a number of target+used
combinations.