[libcxx][libcxxabi][cmake] Deduplicate sanitizer flags function (#221757)

While experimenting with flang-rt builds I realised there were two
copies of this function.
We don't need it in flang-rt right now but I figure reducing duplication
is good anyway.

GitOrigin-RevId: 6dc6c67b786260486641eb62546c34b48143e131