gnu: rust: Add rust 1.54 and move all non-bootstrapping logic to it.

* gnu/packages/rust.scm (rust-1.44): Do not replace LLVM version, for
simplicity.  It used to be required at this point because LLVM 7 was no longer
supported for rust-1.44, but we're now using LLVM 9.
(rust-1.46): Move all additional phases, doc output to rust-1.54.
(rust-1.53, rust-1.54): New variables.
(rust-1.54): Adjust snippet field.
(rust): Set default Rust to the latest version, 1.54.0.
[native-inputs]: Use the latest GDB.

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