The patch adds new "target_version" function attribute and expands existing
"target_clones" one. This is initial implementation of Function Multi Versioning for AArch64
target following Arm C Language Extensions specification ( currently in Beta state )
https://github.com/ARM-software/acle/blob/main/main/acle.md#function-multi-versioning
It is not final yet, any comments, usage feedback, suggestions for improving are very welcome!