[compiler-rt] [compiler-rt] Disable building sanitizers on platforms without fork(). (PR #83485)

[Nico Weber via llvm-commits]

https://github.com/nico approved this pull request.

I think this is a good approach (after addressing the below comment).

>From what I can tell, we only need `fork()` to invoke the external llvm-symbolizer. Maybe in the future we could just not do that on systems where `fork()` isn't available and require people to symbolize on the host machine. But that can be a separate PR done by someone else at some point, and until then this seems like a good improvement to me :)

(I'll wait a day or two for @delcypher to comment as well.)

https://github.com/llvm/llvm-project/pull/83485