[lld] [lld][AArch64] Add support for GCS (PR #90732)

[John Brawn via llvm-commits]

john-brawn-arm wrote:

> Thanks for the patch. Looks quite good.
> 
> I've a question about the alias: `-z gcs is the same as -z gcs=always.`
> 
> Is there a strong need for it? It's quite uncommon to have `-z` option aliases and I'd avoid if there is no compatibility concern.

``-z gcs`` is there to be consistent will all the other similar options to do with the GNU_PROPERTY bits, where there's a ``-z thing`` option which forces the GNU_PROPERTY_THING bit to be set.

https://github.com/llvm/llvm-project/pull/90732