add target feature ABI checks for SPARC

[RalfJung]

Fixes #157083
MCP: rust-lang/compiler-team#1028

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?

View changes since the review

[RalfJung]

I filed an MCP: rust-lang/compiler-team#1028

[fee1-dead]

@rustbot reroll

[chenyukang]

@rustbot reroll

[hanna-kruppe]

I don't really know anything about SPARC nor have I touched the compiler machinery for these checks yet. Maybe one of the people pinged earlier wants to take this? If not, I guess that's my chance to learn some more funny cursed trivia, but it'll take a while.

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These commits modify compiler targets.
(See the Target Tier Policy.)

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This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

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