[scudo] Avoid deprecated-volatile warning in HybridMutex::delayLoop (llvm#67135)

That can prevent compilation with -Werror and -std=c++20:
mutex.h:63:7: error: increment of object of volatile-qualified type 'volatile u32' (aka 'volatile unsigned int') is deprecated [-Werror,-Wdeprecated-volatile]

Author: fabio-d

compiler-rt/lib/scudo/standalone/mutex.h

@@ -59,8 +59,10 @@ class CAPABILITY("mutex") HybridMutex {
     // fast operations.
     constexpr u32 SpinTimes = 16;
     volatile u32 V = 0;
-    for (u32 I = 0; I < SpinTimes; ++I)
-      ++V;
+    for (u32 I = 0; I < SpinTimes; ++I) {
+      u32 Tmp = V + 1;
+      V = Tmp;
+    }
   }
 
   void assertHeldImpl();