scripts/versions.awk: Add local: * to all version nodes

This requires that all exported symbol versions are listed in
Versions files.  It results in more consistent behavior across
architectures because previously, symbols could be exported
via explicit versioned_symbol and compat_symbol macros if the
version node existed in some Versions file (without listing the
symbol), and it was not the base version for the library (which
already had the local: * directive).

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