Auto merge of #121564 - ojeda:rustc-jobserver, r=ehuss

rustc: document the jobserver

Explicitly document that the jobserver may be used by `rustc`, as well as recommend the `+` indicator for integration of `rustc` into GNU Make.

In particular, show the warning to increase the chances that this document is found when searching for solutions online.

In addition, add a note about the issue with GNU Make 4.3 since it is important that users realize they should do this even if they do not expect parallelism from `rustc`.

Finally, show how to workaround the issue of `$(shell ...)` calls in recursive Make (which e.g. was needed for the Linux kernel).

The GNU Make 4.4 case under `--jobserver-style=pipe` is not added since it got fixed after Rust 1.76.0 already (i.e. `rustc` will not warn if it finds the negative file descriptors).

From: rust-lang/rust#120515
Cc: `@petrochenkov` `@belovdv` `@weihanglo` `@bjorn3`

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v2: To be able to use tab characters for the Make examples, add `<!-- ignore-tidy-{check} -->` support to `tidy`.
v3: Added "Integration with build systems" section to hold the GNU Make one. Added "by clearing the `MAKEFLAGS` variable". Added "aforementioned" so that it is clear we are talking about the warning above.
v4: Added CMake subsection. Added a note that `rustc` may be affected by other flags, e.g. `CARGO_MAKEFLAGS`.
v5: Added that `rustc` will choose the number of jobs if a jobserver is not passed.

Author: bors