Added x86_64 System V ABI extern(D) TODO note.
extern(D): Return static arrays via sret parameters.

On x86_64, a proper solution (see GitHub #120) is still needed,
but this will have to wait until the special case for extern(D)
is gone from the implementation.

We need this right now as std.digest.md in 2.061 triggers a
miscompilation issue in the LLVM x86 backend (not the optimizer!)
when returning them directly as LLVM arrays.