Provide a generic PAL feature detection mechanism.

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

src/pal/pal.h

@@ -3,6 +3,23 @@
 namespace snmalloc
 {
   void error(const char* const str);
+
+  /**
+   * Flags in a bitfield of optional features that a PAL may support.  These
+   * should be set in the PAL's `pal_features` static constexpr field.
+   */
+  enum PalFeatures : uint64_t
+  {
+    /**
+     * This PAL supports low memory notifications.  It must implement a
+     * `low_memory_epoch` method that returns a `uint64_t` of the number of
+     * times that a low-memory notification has been raised and an
+     * `expensive_low_memory_check()` method that returns a `bool` indicating
+     * whether low memory conditions are still in effect.
+     */
+    LowMemoryNotification = (1 << 0)
+  };
+
 }
 
 // If simultating OE, then we need the underlying platform

src/pal/pal_free_bsd_kernel.h

@@ -25,10 +25,10 @@ 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;
     void error(const char* const str)
     {
       panic("snmalloc error: %s", str);

src/pal/pal_freebsd.h

@@ -15,10 +15,10 @@ 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);

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);

src/pal/pal_open_enclave.h

@@ -15,10 +15,10 @@ 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)
     {
       UNUSED(str);

src/pal/pal_windows.h

@@ -65,9 +65,10 @@ namespace snmalloc
       }
     }
     /**
-     * Flag indicating that this PAL supports the low pressure notification.
+     * Bitmap of PalFeatures flags indicating the optional features that this
+     * PAL supports.  This PAL supports low-memory notifications.
      */
-    static constexpr bool supports_low_memory_notification = true;
+    static constexpr uint64_t pal_features = LowMemoryNotification;
     /**
      * Counter values for the number of times that a low-pressure notification
      * has been delivered.  Callers should compare this with a previous value