Fixing cpplint readability/casting issues

Change-Id: I6821da0e23737995a9b884a04e9b63fac640cd05
diff --git a/runtime/atomic.cc b/runtime/atomic.cc
index f2a9982..c91db79 100644
--- a/runtime/atomic.cc
+++ b/runtime/atomic.cc
@@ -34,7 +34,7 @@
 static std::vector<Mutex*>* gSwapMutexes;
 
 static Mutex& GetSwapMutex(const volatile int64_t* addr) {
-  return *(*gSwapMutexes)[((unsigned)(void*)(addr) >> 3U) % kSwapMutexCount];
+  return *(*gSwapMutexes)[(reinterpret_cast<unsigned>(addr) >> 3U) % kSwapMutexCount];
 }
 #endif