rustdoc-json: Output target feature information
`#[target_feature]` attributes refer to a target-specific list of features. Enabling certain features can imply enabling other features. Certain features are always enabled on certain targets, since they are required by the target's ABI. Features can also be enabled indirectly based on other compiler flags.
Feature information is ultimately known to `rustc`. Rather than force external tools to track it – which may be wildly impractical due to `-C target-cpu` – have `rustdoc` output `rustc`'s feature data.
This change is motivated by https://github.com/obi1kenobi/cargo-semver-checks/issues/1246, which intends to detect semver hazards caused by `#[target_feature]`.
try-job: aarch64-gnu
try-job: armhf-gnu
try-job: test-various
try-job: x86_64-msvc-1
try-job: i686-msvc-1
try-job: x86_64-mingw-1
try-job: aarch64-apple
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