Templatize the Tag ID value

src/pal/pal.h

@@ -27,7 +27,7 @@ namespace snmalloc
 #  if defined(_WIN32)
     PALWindows;
 #  elif defined(__APPLE__)
-    PALApple;
+    PALApple<>;
 #  elif defined(__linux__)
     PALLinux;
 #  elif defined(FreeBSD_KERNEL)

src/pal/pal_apple.h

@@ -9,11 +9,9 @@ namespace snmalloc
 {
   /**
    * PAL implementation for Apple systems (macOS, iOS, watchOS, tvOS...).
-   *
-   * XNU behaves exactly like a generic BSD platform but this class exists
-   * as a place to add XNU-specific behaviour later, if required.
    */
-  class PALApple : public PALBSD<PALApple>
+  template<int PALAnonID = PALAnonDefaultID>
+  class PALApple : public PALBSD<PALApple<>>
   {
   public:
     /**
@@ -84,7 +82,7 @@ namespace snmalloc
      * (e.g. LLVM sanitizers has 99) so we can monitor their states
      * via vmmap for instance.
      */
-    static constexpr int pal_anon_id = VM_MAKE_TAG(241);
+    static constexpr int pal_anon_id = VM_MAKE_TAG(PALAnonID);
   };
 } // namespace snmalloc
 #endif

src/pal/pal_consts.h

@@ -48,4 +48,9 @@ namespace snmalloc
      */
     YesZero
   };
+
+  /**
+   * Default Tag ID for the Apple class
+   */
+  static const int PALAnonDefaultID = 241;
 } // namespace snmalloc