docs: Cargo.toml - Update rustc dep upstream reference

[polarathene]

UPDATE: Looking at alexcrichton/dlmalloc-rs vs sunfishcode/rustix-dlmalloc history, it seems you're maintaining the fork for rustix support by rewriting history with a rebase to sync for updates? I'll raise this PR upstream at alexcrichton/dlmalloc-rs instead then 👍 (EDIT: Done)

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This appears to have changed in 2020 with Rust 1.47.

The reference doesn't quite communicate the context for usage here in this crate? I assume it's related to building the std lib with nightly? The comment might benefit from a little more context regarding that but I doubt most readers care (if they don't grok the provided context)? 😅

[polarathene]

Using git blame shows the addition was contributed in this commit from Nov 2018, which then references upstream for details: rust-lang/rust#56092

That reference is a bit more clearer with intent.

gimli-rs/addr2line has the following commentary for their Cargo.toml:

[dependencies]
# ...

# Internal feature, only used when building as part of libstd, not part of the
# stable interface of this crate.
core = { version = '1.0.0', optional = true, package = 'rustc-std-workspace-core' }
alloc = { version = '1.0.0', optional = true, package = 'rustc-std-workspace-alloc' }


[features]
# ...

# Internal feature, only used when building as part of libstd, not part of the
# stable interface of this crate.
rustc-dep-of-std = ['core', 'alloc', 'gimli/rustc-dep-of-std']

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That same Nov 2018 commit shows that the cfg-if dep is unrelated to other two deps for supporting rustc-dep-of-std, so it shouldn't be grouped contextually with those 😓

cfg-if was added here (commit from March 2022) and PR in Jan 2024 at alexcrichton/dlmalloc-rs (prior to forking into sunfishcode/rustix-dlmalloc).

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Additionally it seems since Rust 1.89 (Aug 2025), there is no longer a need for compiler_builtins, only core?

Looking at some projects that dropped it, I didn't see any change to a rust-version field in Cargo.toml, so AFAIU this is only relevant to nightly? (unless at some point the ability to build libstd becomes stable).