Make sure memcpy/memmove/memset with size 0 behave correctly

Zero-sized memory accesses are now always permitted, even if the pointer is NULL or dangling (but it must be aligned still). For codegen this means in particular that memcpy/memmove/memset must be lowered to operations that are never UB when the size is 0 (and the pointer is sufficiently aligned). In LLVM that's easy as LLVM's corresponding intrinsics explicitly allow size 0. However, in C, memcpy/memmove/memset with size 0 is UB on NULL (and dangling pointers are impossible to even mention in C), so GCC may use a different semantics for its builtins. For Rust's GCC backend, it's crucial that we use GCC builtins that allow size 0 with any pointer.