Introduce <uk/_types.h> header for basic types defined by Unikraft

Introduce <uk/_types.h> header for basic types defined by Unikraft

Newlib defines some fundamental data types in its <sys/_types.h> header. We need
to overwrite those definitions that are provided by Unikraft so we introduce the
<uk/_types.h> header. In order to use both our <uk/_types.h> and Newlib's
<sys/_types.h> we use an intermediary <sys/_types.h> header in which we can also
overwrite some other type definitions (e.g. _CLOCK_T_ in this patch).

Signed-off-by: Costin Lupu <costin.lupu@cs.pub.ro>
Reviewed-by: Felipe Huici <felipe.huici@neclab.eu>