Report time spent in the linker for binary targets separately

[petr-tik]

Problem

As a a developer working on large executables,
I would like cargo to report the time spent linking executables
so that I can make use the linker that gives me the shortest edit-build-run-tests feedback loop

Currently, i can synthetically achieve that with a hyperfine command that deletes the executable and re-runs cargo build using the total wall-time as an approximation of build time. However, I cannot know how much time it takes cargo to analyse the target/ dir and realise that only the link-step needs to be re-run.

For clean builds I have even less information about link-time alone.

Proposed Solution

1. Start measuring the duration of the linker step for TargetKind::Bin

Add an optional field to TimingInfo in src/cargo/util/machine_message.rs similar to rmeta_time

    #[serde(skip_serializing_if = "Option::is_none")]
    pub link_time: Option<f64>,

Which is only populated for targets that required linking.

2. In the timings report, separate linking from other build steps

Choose a colour for html and a field for JSON and make it visually obvious to people

Notes

Implementing this enables generating the "Use a different linker" actionable suggestion proposed here
#10542

[epage]

Unstable support for this is being added in #15923.