dlfcn: Avoid one-element flexible array in Dl_serinfo [BZ #24166]

dlfcn: Avoid one-element flexible array in Dl_serinfo [BZ #24166]

The dls_serpath path field, as an array of length 1, introduces
unexpected array subscript checks with some compilers.

GCC versions before 3.0 treat the nested anonymous union as a
declaration of an unnamed type, and not as a member declaration,
so this construct cannot be used for these compilers.