add target feature ABI checks for SPARC

[RalfJung]

Fixes #157083

Cc @psumbera @kulikjak @workingjubilee @taiki-e @jonathanpallant I hope this makes sense :)

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[RalfJung]

Ah, the sparc-unknown-linux-gnu target enables v8plus... that's a bit surprising given that I was told that the ABI isn't actually implemented in LLVM yet?
This is also a tier 3 target without a listed target maintainer, so I feel inclined to just remove the target feature from the target spec.

[RalfJung]

Do we need any kind of process for a new cfg value here?

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