[compiler-rt] [tsan] Test `__tsan_test_only_on_fork` only on Mac (PR #96597)

[Arthur Eubanks via llvm-commits]

aeubanks wrote:

we're seeing the following on our mac bots:
```
 /Volumes/Work/s/w/ir/cache/builder/src/third_party/llvm/compiler-rt/test/tsan/Darwin/fork_deadlock.cpp:8:10: fatal error: 'syscall.h' file not found
     8 | #include "syscall.h"
       |          ^~~~~~~~~~~
 1 error generated.
```

https://github.com/llvm/llvm-project/pull/96597