[SME] Add __arm_agnostic("sme_za_state") keyword attribute

[sdesmalen-arm]

The __arm_agnostic keyword attribute enables the user to specify that a function is agnostic to a specified piece of architectural state. That means that the function must preserve this state when it exists, or otherwise ignores its contents.

The reason for not naming this something like __arm_za_compatible was so that we might want use the attribute keyword for other architectural state in the future.

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[sdesmalen-arm]

@rsandifo-arm

[sdesmalen-arm]

The previous comment about updating the inline asm documentation still stands.

Thanks, I missed that comment previously. I've now added a comment that I believe is sufficient to cover the agnostic-ZA case, but let me know if there's still anything missing!