Under the JRE, use java.util.concurrent.atomic.LongAdder unconditionally and (except when we need to support GWT/J2CL) directly instead of through our LongAddable interfaces.

It's available [as of Java 8](https://docs.oracle.com/en/java/javase/23/docs/api/java.base/java/util/concurrent/atomic/LongAdder.html).

This eliminates our [usages of `Unsafe`](#6806) in `Striped64`, a part of the implementation of our `LongAdder`.

On the Android side, we'll have to wait [until we require API Level 24](https://developer.android.com/reference/java/util/concurrent/atomic/LongAdder).

RELNOTES=n/a
PiperOrigin-RevId: 713268966

Author: cpovirk

guava-gwt/src-super/com/google/common/cache/super/com/google/common/cache/LongAddables.java

@@ -23,6 +23,22 @@
  */
 final class LongAddables {
   public static LongAddable create() {
-    return new LongAdder();
+    return new GwtLongAddable();
+  }
+
+  private static final class GwtLongAddable implements LongAddable {
+    private long value;
+
+    public void increment() {
+      value++;
+    }
+
+    public void add(long x) {
+      value += x;
+    }
+
+    public long sum() {
+      return value;
+    }
   }
 }

guava-gwt/src-super/com/google/common/cache/super/com/google/common/cache/LongAdder.java

@@ -15,60 +15,18 @@
 package com.google.common.cache;
 
 import com.google.common.annotations.GwtCompatible;
-import com.google.common.base.Supplier;
-import java.util.concurrent.atomic.AtomicLong;
+import java.util.concurrent.atomic.LongAdder;
 
 /**
- * Source of {@link LongAddable} objects that deals with GWT, Unsafe, and all that.
+ * Source of {@link LongAddable} objects that deals with GWT and all that.
  *
  * @author Louis Wasserman
  */
 @GwtCompatible(emulated = true)
 final class LongAddables {
-  private static final Supplier<LongAddable> SUPPLIER;
-
-  static {
-    Supplier<LongAddable> supplier;
-    try {
-      // trigger static initialization of the LongAdder class, which may fail
-      LongAdder unused = new LongAdder();
-      supplier =
-          new Supplier<LongAddable>() {
-            @Override
-            public LongAddable get() {
-              return new LongAdder();
-            }
-          };
-    } catch (Throwable t) { // we really want to catch *everything*
-      supplier =
-          new Supplier<LongAddable>() {
-            @Override
-            public LongAddable get() {
-              return new PureJavaLongAddable();
-            }
-          };
-    }
-    SUPPLIER = supplier;
-  }
-
   public static LongAddable create() {
-    return SUPPLIER.get();
+    return new JavaUtilConcurrentLongAdder();
   }
 
-  private static final class PureJavaLongAddable extends AtomicLong implements LongAddable {
-    @Override
-    public void increment() {
-      getAndIncrement();
-    }
-
-    @Override
-    public void add(long x) {
-      getAndAdd(x);
-    }
-
-    @Override
-    public long sum() {
-      return get();
-    }
-  }
+  private static final class JavaUtilConcurrentLongAdder extends LongAdder implements LongAddable {}
 }