libc/ctype: Migrate to upstream musl

The files under libc/ctype were added in commit 5cd0168 ("add the string
library from musl") and they have not been modified since. The imported
version is unknown but the difference to our musl git submodule are
cosmetic.

Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>