add GNU gABI

[mildsunrise]

thanks for this list, it has been very useful to me!

this PR mentions the more recent GNU gABI specification, which is NOT Linux-specific but in use by GNU OSes in general. not to be confused with the Linux gABI, which was already mentioned.

given that this has become the de facto standard everywhere in some aspects such as DT_HASH_LINK, and is independent of architectures and OSes, I thought it would be good to mention it right after the System V gABI.

the forum for this ABI was already mentioned down in the GNU/Linux-specific section, by the way. it seems both ABIs are discussed there.

claims are sourced from this article: https://maskray.me/blog/2022-08-21-glibc-and-dt-gnu-hash