Appease gcc-8 warnings (#869)

Summary:
GCC 8 introduces a new `class-memaccess` warning. The compiler rightly complains about `memcpy`-ing a non-POD datatype even though we know it's ok. Casting to `void*` before `memcpy` silences the compiler warning.
Closes https://github.com/facebook/folly/pull/869

Reviewed By: simpkins

Differential Revision: D8475831

Pulled By: danobi

fbshipit-source-id: c0f4eb9a653803cc912be1c6148f50ab71a8c6ab

folly/synchronization/AtomicStruct.h

@@ -68,13 +68,13 @@ class AtomicStruct {
     // we expect the compiler to optimize away the memcpy, but without
     // it we would violate strict aliasing rules
     Raw d = 0;
-    memcpy(&d, &v, sizeof(T));
+    memcpy(&d, static_cast<void*>(&v), sizeof(T));
     return d;
   }
 
   static T decode(Raw d) noexcept {
     T v;
-    memcpy(&v, &d, sizeof(T));
+    memcpy(static_cast<void*>(&v), &d, sizeof(T));
     return v;
   }
 

folly/system/ThreadName.cpp

@@ -195,7 +195,7 @@ bool setThreadName(pthread_t pid, StringPiece name) {
   // std::thread::native_handle_type, which means we can do unsafe things to
   // extract it.
   std::thread::id id;
-  std::memcpy(&id, &pid, sizeof(id));
+  std::memcpy(static_cast<void*>(&id), &pid, sizeof(id));
   return setThreadName(id, name);
 #endif
 }