[libc++] Fix a few warnings in system headers with GCC

This isn't fixing all of them, but at least it's making some progress.

Differential Revision: https://reviews.llvm.org/D106283

NOKEYCHECK=True
GitOrigin-RevId: 2e4755ff6058b5032b77ccca04aa6dab7db081a5
diff --git a/include/memory b/include/memory
index 4f73a81..fdb8943 100644
--- a/include/memory
+++ b/include/memory
@@ -796,7 +796,7 @@
     ptrdiff_t _Np = __end1 - __begin1;
     __end2 -= _Np;
     if (_Np > 0)
-        _VSTD::memcpy(__end2, __begin1, _Np * sizeof(_Tp));
+        _VSTD::memcpy(static_cast<void*>(__end2), static_cast<void const*>(__begin1), _Np * sizeof(_Tp));
 }
 
 struct __destruct_n