[test][compiler-rt] Mark fork_deadlock.cpp UNSUPPORTED

This test is broken since there's no "syscall.h". As requested in #96597, mark test as UNSUPPORTED instead of reverting.

Author: aeubanks

compiler-rt/test/tsan/Darwin/fork_deadlock.cpp

@@ -1,3 +1,6 @@
+// TODO(#96597): fix and reenable
+// UNSUPPORTED: darwin
+
 // RUN: %clangxx_tsan -O1 %s -o %t && %env_tsan_opts=atexit_sleep_ms=0 %run %t 2>&1 | FileCheck %s
 
 // This test models what happens on Mac when fork