Expose `target_abi = "v8plus"` on sparc-unknown-linux-gnu

[RalfJung]

Proposal

(Disclaimer: I am not a SPARC expert, this is my summary based om what I learned in rust-lang/rust#157083 and rust-lang/rust#160562.)

32bit SPARC has (at least) two different ABIs: V8 (used by default by LLVM) and V8+. In such a situation we usually distinguish the ABI by setting target_abi (so the user can cfg on it should that be needed). So I propose we add a new possible value to target_abi: "v8plus" to indicate V8+. We could possibly also add "v8" for the standard V8 ABI, if we want to not leave this as an implicit default.

Cc @taiki-e @jonathanpallant

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[tgross35]

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