[lldb][AArch64] Add release notes and documentation for SME

[DavidSpickett]

Summary

This adds a release note for all the SME support now in LLDB
and a page where I have documented the user experience (for want
of a better term) when using SVE and SME.

This includes things like which mode transitions can or cannot
be triggered from within LLDB. I hope this will serve to A: document
what I've implemented and B: be a user's guide to these extensions.

(though it is not a design document, read the commits and code
for that sort of detail)

[DavidSpickett]

@jasonmolenda you asked some good questions about all this before, any of those not answered by this doc? No need to look deeply into anything, just stuff that comes to mind.

[DavidSpickett]

And if one of those is why does lldb use vg with these strange units, I think "vg" the term Arm's Fast Models used and GDB adopted it. It's possible we could improve that later but for now it is what it is.

[jasonmolenda]

LGTM, fwiw.

lldb/docs/use/aarch64-linux.rst

38

You describe vg without noting the size, but note the size of vq and vl. Maybe these could be described similarly, as counting doublewords/quadwords/bytes.

40

drop second elsewhere

116

architecturally

[DavidSpickett]

• Correct speeling.
• Drop repeated word.
• Clarify vg/vq/vl explanation and end with a listing of each value for the example shown.

[DavidSpickett]

LGTM, fwiw.

Thanks! I wanted to have someone who wasn't knee deep in this to look it over.

[DavidSpickett]

Rebase

[DavidSpickett]

Rebase

[DavidSpickett]

Rebase

[DavidSpickett]

Rebase

[DavidSpickett]

Rebase

[DavidSpickett]

Moved to https://github.com/llvm/llvm-project/pull/66767.