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 :)

[rustbot]

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

[rustbot]

r? @fee1-dead

rustbot has assigned @fee1-dead.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

Why was this reviewer chosen?

The reviewer was selected based on:

• Owners of files modified in this PR: compiler
• compiler expanded to 75 candidates
• Random selection from 20 candidates

[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.

[rustbot]

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

[rustbot]

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

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[RalfJung]

@hanna-kruppe MCP finished, now only the review is left.

Or maybe one of the folks reading along could do that (@workingjubilee @tgross35 ?)

One point that was not fully resolved in the MCP is "v8plus" vs "v8+" for the cfg(target_abi) value. I don't really care either way, it seems hardly anyone else cares either. The only person that expressed a preference was @tgross35 who preferred "v8+". Do the target maintainers have an opinion?

[jonathanpallant]

AFAIK this only applies to sparc-unknown-linux-gnu right? There is no target maintainer, nor platform docs page. So I think you can do whatever you like.

[RalfJung]

Oh, right.