Store on vec may take pointer to constant type

The `store` member function on `sycl::vec` takes a `multi_ptr` argument without considering whether or not `DataT` is `const`. Since the function intends to write to the pointer this seems unintentional. If this is intentional, this function is affected by the same problem as described in https://github.com/KhronosGroup/SYCL-Docs/issues/540.

Possible solution: Change the signature to:

```c++
template
void store(size_t offset, multi_ptr, AddressSpace, IsDecorated> ptr) const;
```

Start by locating the sycl::vec::store declaration in the specification sources and review the linked SYCL-Docs issue 540 for the related const behavior. Confirm the intended multi_ptr signature from the issue, then update the documented API so the completed signature consistently handles const DataT.

Written by the indexing model from the issue text.