Provide a generic PAL feature detection mechanism.

We can support up to 64 optional features, currently only one is used.

src/pal/pal_linux.h

@@ -14,10 +14,10 @@ namespace snmalloc
   {
   public:
     /**
-     * Flag indicating that this PAL does not support low pressure
-     * notifications.
+     * Bitmap of PalFeatures flags indicating the optional features that this
+     * PAL supports.
      */
-    static constexpr bool supports_low_memory_notification = false;
+    static constexpr uint64_t pal_features = 0;
     static void error(const char* const str)
     {
       puts(str);