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

[Nico Weber via llvm-commits]

nico wrote:

Does having the sanitizers runtimes help you if you can't use them though?

As mentioned further up, the `fork` isn't really needed for anything – is someone wanted to build them, the nicer fix seems to not run `llvm-symbolizer` on-device on these platforms, and then there's no need for `fork`.

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