Make TSan runtime initialization and finalization hooks work on Darwin
even if these hooks are not built in the main executable. When these
hooks are defined in another library that is not directly linked against
the TSan runtime (e.g., Swift runtime) we cannot rely on the "strong-def
overriding weak-def" mechanics and have to look them up via `dlsym()`.
Let's also define hooks that are easier to use from C-only code:
extern "C" void __tsan_on_initialize();
extern "C" bool __tsan_on_finalize(bool failed);
For now, these will call through to the old hooks. Eventually, we want
to adopt the new hooks downstream and remove the old ones.
This is part of the effort to support Swift Tasks (async/await and
actors) in TSan.