Add riscv32im-risc0-zkvm-elf target (corresp. rust-lang/rust PR 117958)

[weikengchen]

This PR proposes adding the riscv32im-risc0-zkvm-elf target. Unlike the zkasm target which may be premature, the risc0 one has been used by developers, can be used to create ZK/blockchain applications for payments to email address (see https://pay.demos.bonsai.xyz/), and is currently undergoing the process of being part of Rust by satisfying the requirement of being a tier-3 target.

rust-lang/rust#117958
(The remaining steps are mostly to sync up with the latest Rust main)

A formal document that describes this tier-3 target can also be found:
https://github.com/rust-lang/rust/blob/8db6f597169b32e7a970ffa66bec5e7dae35d8a8/src/doc/rustc/src/platform-support/riscv32im-risc0-zkvm-elf.md

The only change needed here is to add zkvm as a valid operating system. Picking zkvm as an operating system rather than an environment is to stay consistent with the Rust tier-3 target configuration as specified here:
https://github.com/rust-lang/rust/pull/117958/files#diff-9d4db4407bd3a1d92dca3c693fd1fce5b9b11cb5726fe92d18aa4d7bd65caae1R12

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I argue that adding this target into target-lexicon is timely, for two reasons.

• The target has been used in production and likely would become part of Rust soon.
• We already have developers that suffer from target-lexicon not knowing this target---a developer (https://twitter.com/NitanshuL) working on using wasmer https://docs.rs/wasmer/latest/wasmer/ would not be able to proceed with this library because wasmer uses target-lexicon, and target-lexicon refuses to be built in an architecture that it does not know (a build.rs thing).

thread 'main' panicked at 'Invalid target name: 'riscv32im-risc0-zkvm-elf'', from target-lexicon-0.12.12/build.rs:52:54

[weikengchen]

I am happy to say that the target has been merged in the upstream.
rust-lang/rust#117958

As a result, this PR might be ready now.

[weikengchen]

(cc @SchmErik on this one)

[SchmErik]

@weikengchen yeah, it's been merged and will be a part of 1.77 so I think this is ready to go

[SchmErik]

Hi @sunfishcode this target triple has landed as a part of rust's tier 3 targets as of 1.77. I'm wondering if there's anything else that you need from our end in order to merge this PR