Merge bitcoin/bitcoin#24372: bench: Avoid deprecated use of volatile +=

9999f891d1 bench: Avoid deprecated use of volatile += (MarcoFalke)

Pull request description:

  Deprecated in C++20 according to https://eel.is/c++draft/expr.ass#6 .

  ```
  bench/examples.cpp:16:13: warning: compound assignment with ‘volatile’-qualified left operand is deprecated [-Wvolatile]
     16 |         sum += sin(d);
        |         ~~~~^~~~~~~~~
  ```

  While C++20 is currently unsupported, I don't see any downside to a minor fixup to an example benchmark. This will also make a hypothetical C++20 patch smaller.

ACKs for top commit:
  fanquake:
    ACK 9999f891d1

Tree-SHA512: ca7d660fa8eba347a4648408a8b97a0ecb8263a825da7abd59129d783058102581e05b273667989f95480436a66d5384bd1e92d9ae79408f5b30e2178935cc38

src/bench/examples.cpp

@@ -13,7 +13,7 @@ static void Trig(benchmark::Bench& bench)
 {
     double d = 0.01;
     bench.run([&] {
-        sum += sin(d);
+        sum = sum + sin(d);
         d += 0.000001;
     });
 }