[clang] [compiler-rt] [rtsan] Enable RealtimeSanitizer for FreeBSD (PR #125389)

[Chris Apple via cfe-commits]

cjappl wrote:

Thanks for the feedback @brooksdavis and @bsdimp - always hard to tell how popular or unpopular a change would be, especially from "outside the community" (aka me not knowing much about the BSD flavors :) 

> Sure we can keep it open for a while, still a long road before 21.x anyway.

Sounds like a plan. 

https://github.com/llvm/llvm-project/pull/125389