Don't generate bootstrap usage unless it's needed

Previously, `x.py` would unconditionally run `x.py build` to get the
help message. After rust-lang#76165,
when checking the CI stage was moved into `Config`, that would cause an
assertion failure (but only only in CI!):

```
thread 'main' panicked at 'assertion failed: `(left == right)`
  left: `1`,
 right: `2`', src/bootstrap/config.rs:619:49
```

This changes bootstrap to only generate a help message when it needs
to (when someone passes `--help`).

r? @Mark-Simulacrum
This should fix the CI failures in rust-lang#76797 and rust-lang#75991.

Don't generate bootstrap usage unless it's needed

Previously, `x.py` would unconditionally run `x.py build` to get the
help message. After rust-lang#76165,
when checking the CI stage was moved into `Config`, that would cause an
assertion failure (but only only in CI!):

```
thread 'main' panicked at 'assertion failed: `(left == right)`
  left: `1`,
 right: `2`', src/bootstrap/config.rs:619:49
```

This changes bootstrap to only generate a help message when it needs
to (when someone passes `--help`).

r? @Mark-Simulacrum
This should fix the CI failures in rust-lang#76797 and rust-lang#75991.

Don't generate bootstrap usage unless it's needed

Previously, `x.py` would unconditionally run `x.py build` to get the
help message. After rust-lang#76165,
when checking the CI stage was moved into `Config`, that would cause an
assertion failure (but only only in CI!):

```
thread 'main' panicked at 'assertion failed: `(left == right)`
  left: `1`,
 right: `2`', src/bootstrap/config.rs:619:49
```

This changes bootstrap to only generate a help message when it needs
to (when someone passes `--help`).

r? @Mark-Simulacrum
This should fix the CI failures in rust-lang#76797 and rust-lang#75991.

Don't generate bootstrap usage unless it's needed

Previously, `x.py` would unconditionally run `x.py build` to get the
help message. After rust-lang#76165,
when checking the CI stage was moved into `Config`, that would cause an
assertion failure (but only only in CI!):

```
thread 'main' panicked at 'assertion failed: `(left == right)`
  left: `1`,
 right: `2`', src/bootstrap/config.rs:619:49
```

This changes bootstrap to only generate a help message when it needs
to (when someone passes `--help`).

r? @Mark-Simulacrum
This should fix the CI failures in rust-lang#76797 and rust-lang#75991.

Don't generate bootstrap usage unless it's needed

Previously, `x.py` would unconditionally run `x.py build` to get the
help message. After rust-lang#76165,
when checking the CI stage was moved into `Config`, that would cause an
assertion failure (but only only in CI!):

```
thread 'main' panicked at 'assertion failed: `(left == right)`
  left: `1`,
 right: `2`', src/bootstrap/config.rs:619:49
```

This changes bootstrap to only generate a help message when it needs
to (when someone passes `--help`).

r? @Mark-Simulacrum
This should fix the CI failures in rust-lang#76797 and rust-lang#75991.

Rollup of 15 pull requests

Successful merges:

 - rust-lang#76722 (Test and fix Send and Sync traits of BTreeMap artefacts)
 - rust-lang#76766 (Extract some intrinsics out of rustc_codegen_llvm)
 - rust-lang#76800 (Don't generate bootstrap usage unless it's needed)
 - rust-lang#76809 (simplfy condition in ItemLowerer::with_trait_impl_ref())
 - rust-lang#76815 (Fix wording in mir doc)
 - rust-lang#76818 (Don't compile regex at every function call.)
 - rust-lang#76821 (Remove redundant nightly features)
 - rust-lang#76823 (black_box: silence unused_mut warning when building with cfg(miri))
 - rust-lang#76825 (use `array_windows` instead of `windows` in the compiler)
 - rust-lang#76827 (fix array_windows docs)
 - rust-lang#76828 (use strip_prefix over starts_with and manual slicing based on pattern length (clippy::manual_strip))
 - rust-lang#76840 (Move to intra doc links in core/src/future)
 - rust-lang#76845 (Use intra docs links in core::{ascii, option, str, pattern, hash::map})
 - rust-lang#76853 (Use intra-doc links in library/core/src/task/wake.rs)
 - rust-lang#76871 (support panic=abort in Miri)

Failed merges:

r? `@ghost`