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/test/func/thread_alloc_external/thread_alloc_external.cc

@@ -0,0 +1,40 @@
+
+#define SNMALLOC_EXTERNAL_THREAD_ALLOC
+#include <mem/globalalloc.h>
+using namespace snmalloc;
+
+class ThreadAllocUntyped
+{
+public:
+  static void* get()
+  {
+    static thread_local void* alloc = nullptr;
+    if (alloc != nullptr)
+    {
+      return alloc;
+    }
+ 
+    alloc = current_alloc_pool()->acquire();
+    return alloc;
+  }
+};
+
+#include <override/malloc.cc>
+
+int main()
+{
+  MemoryProviderStateMixin<DefaultPal> mp;
+
+  // 28 is large enough to produce a nested allocator.
+  // It is also large enough for the example to run in.
+  // For 1MiB superslabs, SUPERSLAB_BITS + 4 is not big enough for the example.
+ 
+  auto a = ThreadAlloc::get();
+
+  for (size_t i = 0; i < 1000; i++)
+  {
+    auto r1 = a->alloc(100);
+ 
+    free(r1);
+  }
+}