Fix memory load ordering bug (#66952)

Co-authored-by: Aaron Robinson <[email protected]>

Author: jkoritzinsky

src/native/libs/System.Native/pal_dynamicload.c

@@ -57,13 +57,18 @@ void* SystemNative_GetDefaultSearchOrderPseudoHandle(void)
     return (void*)RTLD_DEFAULT;
 }
 #else
-static void* g_defaultSearchOrderPseudoHandle = NULL;
+static void* volatile g_defaultSearchOrderPseudoHandle = NULL;
 void* SystemNative_GetDefaultSearchOrderPseudoHandle(void)
 {
-    if (g_defaultSearchOrderPseudoHandle == NULL)
+    // Read the value once from the volatile static to avoid reading from memory twice.
+    void* defaultSearchOrderPseudoHandle = (void*)g_defaultSearchOrderPseudoHandle;
+    if (defaultSearchOrderPseudoHandle == NULL)
     {
-        g_defaultSearchOrderPseudoHandle = dlopen(NULL, RTLD_LAZY);
+        // Assign back to the static as well as the local here.
+        // We don't need to check for a race between two threads as the value returned by
+        // dlopen here will always be the same in a given environment.
+        g_defaultSearchOrderPseudoHandle = defaultSearchOrderPseudoHandle = dlopen(NULL, RTLD_LAZY);
     }
-    return g_defaultSearchOrderPseudoHandle;
+    return defaultSearchOrderPseudoHandle;
 }
 #endif