Reduce dependence on C++ runtime

If the external thread statics are used, then
we don't need to include some C++ runtime
concepts. This refactoring moves some global initialization under
conditional compilation.

src/pal/pal_open_enclave.h

@@ -11,7 +11,7 @@ namespace snmalloc
 {
   class PALOpenEnclave
   {
-    std::atomic<void*> oe_base;
+    std::atomic<void*> oe_base = nullptr;
 
   public:
     /**