Constify fmt::from_fn

[EFanZh]

See #117729 (comment).

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[rust-log-analyzer]

The job aarch64-gnu-llvm-20-1 failed! Check out the build log: (web) (plain enhanced) (plain)

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[RUSTC-TIMING] build_script_build test:false 0.314
error: function has missing const stability attribute
    --> library/core/src/fmt/builders.rs:1231:1
     |
1231 | pub const fn from_fn<F: Fn(&mut fmt::Formatter<'_>) -> fmt::Result>(f: F) -> FromFn<F> {
     | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

[RUSTC-TIMING] core test:false 23.984
error: could not compile `core` (lib) due to 1 previous error
Bootstrap failed while executing `--stage 2 test --skip compiler --skip src`