[dogfooding] `-Zbuild-std -vv` produces lots of warnings about check-cfg

[tesuji]

With extra verbose flag (-vv), one would see a lots of warnings about check-cfg when building -Zbuild-std.
But it doesn't need to be that way.

Can we doodfooding check-cfg flag to Cargo.toml and build.rs to resolve them?

Step to reproduce:

cargo new dummy && cd dummy
cargo build -Zbuild-std=core --target=x86_64-unknown-linux-gnu -vv

I expected to see this happen: No warnings about " unexpected cfg"

Some are:

2024-05-19T18:46:23.3979433Z warning: unexpected `cfg` condition name: `no_fp_fmt_parse`
2024-05-19T18:46:23.3980468Z   --> /home/runner/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/num/mod.rs:57:11
2024-05-19T18:46:23.3981241Z    |
2024-05-19T18:46:23.3981518Z 57 | #[cfg(not(no_fp_fmt_parse))]
2024-05-19T18:46:23.3981960Z    |           ^^^^^^^^^^^^^^^
2024-05-19T18:46:23.3982267Z    |
2024-05-19T18:46:23.3983114Z    = help: consider using a Cargo feature instead or adding `println!("cargo::rustc-check-cfg=cfg(no_fp_fmt_parse)");` to the top of the `build.rs`
2024-05-19T18:46:23.3984658Z    = note: see <https://doc.rust-lang.org/nightly/cargo/reference/build-scripts.html#rustc-check-cfg> for more information about checking conditional configuration
2024-05-19T18:46:23.3985452Z 
2024-05-19T18:46:23.3999470Z warning: unexpected `cfg` condition name: `bootstrap`
2024-05-19T18:46:23.4001329Z    --> /home/runner/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/intrinsics.rs:990:16
2024-05-19T18:46:23.4002762Z     |
2024-05-19T18:46:23.4003512Z 990 | #[cfg_attr(not(bootstrap), miri::intrinsic_fallback_checks_ub)]
2024-05-19T18:46:23.4004371Z     |                ^^^^^^^^^
2024-05-19T18:46:23.4004969Z     |
2024-05-19T18:46:23.4006329Z     = help: consider using a Cargo feature instead or adding `println!("cargo::rustc-check-cfg=cfg(bootstrap)");` to the top of the `build.rs`
2024-05-19T18:46:23.4008775Z     = note: see <https://doc.rust-lang.org/nightly/cargo/reference/build-scripts.html#rustc-check-cfg> for more information about checking conditional configuration
2024-05-19T18:46:23.4010415Z 

@rustbot label +F-check-cfg +-Zbuild-std
cc @Urgau