[XRay][compiler-rt][Darwin] Use dynamic initialisation as an alternative

[dberris]

Summary

In cases where we can't use the .preinit_array section (as in Darwin for
example) we instead use dynamic initialisation. We know that this
alternative approach will race with the initializers of other objects at
global scope, but this is strictly better than nothing.

lib/xray/xray_init.cc

__sanitizer::atomic_store(&XRayInitialized, true,
                             __sanitizer::memory_order_release);
 
 #ifndef XRAY_NO_PREINIT
   if (flags()->patch_premain)
     __xray_patch();
 #endif
 }
 
-// Only add the preinit array initialization if the sanitizers can.
 #if !defined(XRAY_NO_PREINIT) && SANITIZER_CAN_USE_PREINIT_ARRAY
+// Only add the preinit array initialization if the sanitizers can.
 __attribute__((section(".preinit_array"),
                used)) void (*__local_xray_preinit)(void) = __xray_init;
+#else
+// If we cannot use the .preinit_array section, we should instead use dynamic
+// initialisation.
+static bool UNUSED __local_xray_dyninit = [] {
+  __xray_init();
+  return true;
+}();
 #endif