musl: more cleanups in stdio/

Remove 4 files that were identical to the musl originals except the
include of <stdio.h> was replaced by either "stdio.h" or "stdio_impl.h".

It is working well with <stdio.h>, so we can use the unmodified musl
files.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
libc: _chk versions for memset and memmove

The needed check should be pretty trivial, it is just a matter of ensuring the
size is respected. To move towards a more common implementation, I am bringing
in here a new function _chk_fail, that can be called when the check fails. I am
not converting any users, but I can do that, if we agree upon this approach. In
any case, because I abort on _chk_fail, the current behavior should be consistent
with existing checks.

Signed-off-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>