buffer: avoid materializing ArrayBuffer for creation

Do not create an `ArrayBuffer` if the engine’s settings avoid it
and we don’t need it.

lib/buffer.js

@@ -105,21 +105,17 @@ let poolSize, poolOffset, allocPool;
 const zeroFill = bindingZeroFill || [0];
 
 function createUnsafeBuffer(size) {
-  return new FastBuffer(createUnsafeArrayBuffer(size));
-}
-
-function createUnsafeArrayBuffer(size) {
   zeroFill[0] = 0;
   try {
-    return new ArrayBuffer(size);
+    return new FastBuffer(size);
   } finally {
     zeroFill[0] = 1;
   }
 }
 
 function createPool() {
   poolSize = Buffer.poolSize;
-  allocPool = createUnsafeArrayBuffer(poolSize);
+  allocPool = createUnsafeBuffer(poolSize).buffer;
   poolOffset = 0;
 }
 createPool();