Linux: Avoid conflicting types in ld.so --list-diagnostics

The path auxv[*].a_val could either be an integer or a string,
depending on the a_type value.  Use a separate field, a_val_string, to
simplify mechanical parsing of the --list-diagnostics output.

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