[PATCH] ntdll: Optimize memcpy for x86-64.

[Elaine Lefler]

Looking into the code duplication some more: msvcrt links with ntdll,
so it seems the simplest solution is to remove the functions from
msvcrt, which will cause the ntdll versions to be called.

It's not as clean as I'd like, though. Some of the functions are
better in msvcrt, so they should be backported, and others can't be
moved at all. This isn't the only source of duplication either. For
instance, math.c seems to be full of duplicates, but I don't even want
to touch that...

If upstream typically doesn't like this kind of patch, maybe I should
resubmit after I've developed an AVX2 patch as well? That would
provide a much more convincing argument for speed, compared to the
current patch which is fairly marginal.