elf: Make the tunable struct definition internal only

The representation of the tunables including type information and
the tunable list structure are only used in the implementation not
in the tunables api that is exposed to usage within glibc.

This patch moves the representation related definitions into the
existing dl-tunable-types.h and uses that only for implementation.

The tunable callback and related types are moved to dl-tunables.h
because they are part of the tunables api.

This reduces the details exposed in the tunables api so the internals
are easier to change.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>