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

[via llvm-commits]

rohit-rao wrote:

@delcypher per discussion on https://discourse.llvm.org/t/refined-controlling-of-options-in-distribution-builds-for-darwin/69976/2, the watchOS/tvOS sanitizer runtimes cannot build against public SDKs. I don't need to build sanitizers locally, but I would like to have the watchOS builtins library, and currently COMPILER_RT_ENABLE_WATCHOS=ON tries to build both (and fails).

Adding a test for `fork()` when evaluating archs should correlate directly with whether or not the sanitizer runtimes will actually build -- this gives us a way to exclude watchOS/tvOS while maintaining support for builtins.

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