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

[Fangrui Song via llvm-commits]

MaskRay 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.

Other options setting GNU_PROPERTY are quite different in names, `-z force-bti`, `-z force-ibt`.

To the best of my knowledge,

* There is no `-z` alias options
* The few aliases linkers have are for `-z` vs non-z options, which might be related to SunOS/Solaris heritage. In newer options there is very few (if any) alias.

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