[flang] explicitly cast the pointer to void* in std::memcpy calls (NFC)

[kkwli]

The clang 20 compiler gives the following errors in descriptor.cpp and assign.cpp.

/scratch/kli/llvm-project/flang-rt/lib/runtime/assign.cpp:266:19: error: first argument in call to 'memcpy' is a pointer to non-trivially copyable type 'Descriptor' [-Werror,-Wnontrivial-memcall]
  266 |       std::memcpy(deferDeallocation, &to, to.SizeInBytes());
      |                   ^
/scratch/kli/llvm-project/flang-rt/lib/runtime/assign.cpp:266:19: note: explicitly cast the pointer to silence this warning
  266 |       std::memcpy(deferDeallocation, &to, to.SizeInBytes());
      |                   ^
      |                   (void*)
/scratch/kli/llvm-project/flang-rt/lib/runtime/assign.cpp:274:19: error: first argument in call to 'memcpy' is a pointer to non-trivially copyable type 'Descriptor' [-Werror,-Wnontrivial-memcall]
  274 |       std::memcpy(&newFrom, &from, descBytes);
      |                   ^
/scratch/kli/llvm-project/flang-rt/lib/runtime/assign.cpp:274:19: note: explicitly cast the pointer to silence this warning
  274 |       std::memcpy(&newFrom, &from, descBytes);
      |                   ^
      |                   (void*)
2 errors generated.

This patch is to add the explicit cast to the first argument of std::memcpy.